build: disable some gcc compier warnings

Add --enable-extra-warning option to the configure script. By default,
it is disabled to suppress some gcc warnings:

* -Wno-misleading-indentation. This is because, for the short term, it
  seems unfeasible to clearly rewrite FORM source code (by hand) unless
  we introduce some automatization with formatters, linters or static
  analyzers.

* -Wno-stringop-overflow. It seems to give many false positives (though
  there may be some true positives).

Stop using obsolete macros when autoconf>=2.70 is used.

Improve the handling of i486 (Pentium) processors on old 32-bit systems
(-march=i486).

ax_check_compile_flag.m4 (GPL-3.0-or-later) is taken from:
  autoconf-archive/autoconf-archive@7f56aaa (v2014.10.15)

Resolve vermaseren#416

configure.ac

@@ -66,8 +66,9 @@ m4_define([serial_tests], [m4_esyscmd_s([
 	awk '{split ($NF,a,"."); if (a[1] >= 2 || (a[1] == 1 && a[2] >= 13)) { print "serial-tests" }}'
 ])])
 
-AC_PREREQ(2.59)
-AC_INIT([FORM], FORM_VERSION, [https://github.com/vermaseren/form/issues])
+AC_PREREQ([2.59])
+AC_INIT([FORM], [FORM_VERSION], [https://github.com/vermaseren/form/issues])
+AC_CONFIG_MACRO_DIR([m4])
 AC_CONFIG_SRCDIR([sources/form3.h])
 AC_CONFIG_HEADERS([config.h])
 AC_CONFIG_AUX_DIR([build-aux])
@@ -96,8 +97,18 @@ AC_PROG_CXX([g++ c++ icpc])
 AC_PROG_LN_S
 
 # Checks for header files
-AC_HEADER_STDC
-AC_HEADER_TIME
+# NOTE: autoconf>=2.70 has started to warn obsolete macros.
+# We check if AC_CHECK_INCLUDES_DEFAULT (introduced in 2.70) is available
+# and switch to the new code if possible.
+m4_ifdef([AC_CHECK_INCLUDES_DEFAULT],
+	[AC_CHECK_INCLUDES_DEFAULT
+	AC_PROG_EGREP
+	AC_CHECK_HEADERS_ONCE([sys/time.h])
+	AC_DEFINE([TIME_WITH_SYS_TIME],
+		[1],
+		[Define to 1 if you can safely include both <sys/time.h> and <time.h>. This macro is obsolete.])],
+	[AC_HEADER_STDC
+	AC_HEADER_TIME])
 AC_CHECK_HEADERS([fcntl.h limits.h sys/file.h])
 
 AC_LANG_PUSH([C++])
@@ -160,17 +171,20 @@ case $host_os in
 		;;
 esac
 case $host_cpu in
-	i?86 )
+	i586 )
 		print_cpu="Pentium"
 		;;
+	i686 )
+		print_cpu="Pentium Pro"
+		;;
 	x86_64 )
 		print_cpu="Opteron"
 		;;
 	alpha* )
 		print_cpu="Alpha"
 		;;
 	* )
-		print_cpu="UNKNOWN CPU"
+		print_cpu="UNKNOWN CPU ($host_cpu)"
 		;;
 esac
 
@@ -730,6 +744,13 @@ AC_ARG_ENABLE([sanitize],
 		[enable_sanitize=no])],
 	[enable_sanitize=no])
 
+# Check for extra compiler warnings
+AC_ARG_ENABLE([extra-warning],
+	[AS_HELP_STRING([--enable-extra-warning],
+		[enable extra compiler warnings @<:@default=no@:>@])],
+	[enable_extra_warning=yes],
+	[enable_extra_warning=no])
+
 # Optimization/debugging flags
 AC_ARG_VAR([COMPILEFLAGS], [Compiler flags for release versions])
 AC_ARG_VAR([LINKFLAGS], [Linker flags for release versions])
@@ -738,36 +759,45 @@ AC_ARG_VAR([DEBUGLINKFLAGS], [Linker flags for debugging versions])
 TOOL_LIBS=
 DEBUGTOOL_LIBS=
 
+# AX_HANDLE_EXTRA_WARNING(COMPILEFLAGS)
+# -------------------------------------
+AC_DEFUN([AX_HANDLE_EXTRA_WARNING],
+[if test "x$enable_extra_warning" = xno; then
+	# Too many misleading indentation in the FORM source code.
+	AX_CHECK_COMPILE_FLAG([-Wno-misleading-indentation],
+		[$1="$$1 -Wno-misleading-indentation"])
+	# Too many false positives.
+	# See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88443
+	AX_CHECK_COMPILE_FLAG([-Wno-stringop-overflow],
+		[$1="$$1 -Wno-stringop-overflow"])
+fi[]dnl
+])
+
 my_test_COMPILEFLAGS=${COMPILEFLAGS+set}
 if test "$my_test_COMPILEFLAGS" != set; then
 	if test "x$vendor" = xgnu; then
 		# We don't use -pedantic option because of horrible warnings.
-		COMPILEFLAGS="-Wall -Wextra -Wpadded -O3"
+		COMPILEFLAGS="-Wall -Wextra -Wpadded"
+		AX_HANDLE_EXTRA_WARNING([COMPILEFLAGS])
+		# Enable optimizations.
+		COMPILEFLAGS="$COMPILEFLAGS -O3"
 		if test "x$enable_profile" != xgprof; then
 			# -pg conflicts with -fomit-frame-pointer.
 			COMPILEFLAGS="$COMPILEFLAGS -fomit-frame-pointer"
 		fi
 		if test "x$enable_native" = xyes; then
-			# Check for -march=native.
-			AC_MSG_CHECKING([whether compiler accepts -march=native])
-			ok=no
-			save_CFLAGS=$CFLAGS
-			CFLAGS="$CFLAGS -march=native"
-			AC_COMPILE_IFELSE([AC_LANG_PROGRAM()], [ok=yes])
-			CFLAGS=$save_CFLAGS
-			AC_MSG_RESULT($ok)
-			if test "x$ok" = xyes; then
-				COMPILEFLAGS="$COMPILEFLAGS -march=native"
-			else
-				if test "x$print_data_model" = xILP32; then
+			# Use -march=native if available.
+			AX_CHECK_COMPILE_FLAG([-march=native],
+				[COMPILEFLAGS="$COMPILEFLAGS -march=native"],
+				[if test "x$print_data_model" = xILP32; then
 					if test "x$print_cpu" = xPentium; then
-						# NOTE: In a strict sense, i686 must be used for Pentium Pro or later.
+						COMPILEFLAGS="$COMPILEFLAGS -march=i586"
+					elif test "x$print_cpu" = 'xPentium Pro'; then
 						COMPILEFLAGS="$COMPILEFLAGS -march=i686"
 					elif test "x$print_cpu" = xOpteron; then
 						COMPILEFLAGS="$COMPILEFLAGS -march=opteron"
 					fi
-				fi
-			fi
+				fi])
 		fi
 		# Profiling option.
 		if test "x$enable_profile" = xgprof; then
@@ -809,24 +839,18 @@ fi
 my_test_DEBUGCOMPILEFLAGS=${DEBUGCOMPILEFLAGS+set}
 if test "$my_test_DEBUGCOMPILEFLAGS" != set && test "x$enable_debug" = xyes; then
 	if test "x$vendor" = xgnu; then
-		DEBUGCOMPILEFLAGS='-g3 -Wall -Wextra'
+		DEBUGCOMPILEFLAGS='-Wall -Wextra'
 		if test "x$enable_sanitize" = xno; then
 			# UBSan puts many paddings (at least in gcc 5.3).
 			DEBUGCOMPILEFLAGS="$DEBUGCOMPILEFLAGS -Wpadded"
 		fi
-		# Check for -Og.
-		AC_MSG_CHECKING([whether compiler accepts -Og])
-		ok=no
-		save_CFLAGS=$CFLAGS
-		CFLAGS="$CFLAGS -Og"
-		AC_COMPILE_IFELSE([AC_LANG_PROGRAM()], [ok=yes])
-		CFLAGS=$save_CFLAGS
-		AC_MSG_RESULT($ok)
-		if test "x$ok" = xyes; then
-			DEBUGCOMPILEFLAGS="$DEBUGCOMPILEFLAGS -Og"
-		else
-			DEBUGCOMPILEFLAGS="$DEBUGCOMPILEFLAGS -O0"
-		fi
+		AX_HANDLE_EXTRA_WARNING([DEBUGCOMPILEFLAGS])
+		# Check for -Og. If it doesn't work then use -O0.
+		AX_CHECK_COMPILE_FLAG([-Og],
+			[DEBUGCOMPILEFLAGS="$DEBUGCOMPILEFLAGS -Og"],
+			[DEBUGCOMPILEFLAGS="$DEBUGCOMPILEFLAGS -O0"])
+		# Debugging information.
+		DEBUGCOMPILEFLAGS="$DEBUGCOMPILEFLAGS -g3"
 		# Coverage option.
 		if test "x$enable_coverage" = xyes; then
 			DEBUGCOMPILEFLAGS="$DEBUGCOMPILEFLAGS -coverage"
@@ -840,31 +864,13 @@ if test "$my_test_DEBUGCOMPILEFLAGS" != set && test "x$enable_debug" = xyes; the
 				else
 					tmp_sanitize=$enable_sanitize,$san
 				fi
-				AC_MSG_CHECKING([whether compiler accepts -fsanitize=$tmp_sanitize])
-				ok=no
-				save_CFLAGS=$CFLAGS
-				CFLAGS="$CFLAGS -fsanitize=$tmp_sanitize"
-				AC_COMPILE_IFELSE([AC_LANG_PROGRAM()], [ok=yes])
-				CFLAGS=$save_CFLAGS
-				AC_MSG_RESULT($ok)
-				if test "x$ok" = xyes; then
-					enable_sanitize=$tmp_sanitize
-				fi
+				AX_CHECK_COMPILE_FLAG([-fsanitize=$tmp_sanitize], [enable_sanitize=$tmp_sanitize])
 			done
 			if test "x$enable_sanitize" = x; then
 				enable_sanitize=failed
 			fi
 		elif test "x$enable_sanitize" != xno; then
-			AC_MSG_CHECKING([whether compiler accepts -fsanitize=$enable_sanitize])
-			ok=no
-			save_CFLAGS=$CFLAGS
-			CFLAGS="$CFLAGS -fsanitize=$enable_sanitize"
-			AC_COMPILE_IFELSE([AC_LANG_PROGRAM()], [ok=yes])
-			CFLAGS=$save_CFLAGS
-			AC_MSG_RESULT($ok)
-			if test "x$ok" != xyes; then
-				enable_sanitize=failed
-			fi
+			AX_CHECK_COMPILE_FLAG([-fsanitize=$enable_sanitize], [], [enable_sanitize=failed])
 		fi
 		if test "x$enable_sanitize" = xfailed; then
 			AC_MSG_FAILURE([test for sanitizer failed. Give --disable-sanitize if you want to compile without sanitizer])

m4/ax_check_compile_flag.m4

@@ -0,0 +1,74 @@
+# ===========================================================================
+#   http://www.gnu.org/software/autoconf-archive/ax_check_compile_flag.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_CHECK_COMPILE_FLAG(FLAG, [ACTION-SUCCESS], [ACTION-FAILURE], [EXTRA-FLAGS], [INPUT])
+#
+# DESCRIPTION
+#
+#   Check whether the given FLAG works with the current language's compiler
+#   or gives an error.  (Warnings, however, are ignored)
+#
+#   ACTION-SUCCESS/ACTION-FAILURE are shell commands to execute on
+#   success/failure.
+#
+#   If EXTRA-FLAGS is defined, it is added to the current language's default
+#   flags (e.g. CFLAGS) when the check is done.  The check is thus made with
+#   the flags: "CFLAGS EXTRA-FLAGS FLAG".  This can for example be used to
+#   force the compiler to issue an error when a bad flag is given.
+#
+#   INPUT gives an alternative input source to AC_COMPILE_IFELSE.
+#
+#   NOTE: Implementation based on AX_CFLAGS_GCC_OPTION. Please keep this
+#   macro in sync with AX_CHECK_{PREPROC,LINK}_FLAG.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Guido U. Draheim <[email protected]>
+#   Copyright (c) 2011 Maarten Bosmans <[email protected]>
+#
+#   This program is free software: you can redistribute it and/or modify it
+#   under the terms of the GNU General Public License as published by the
+#   Free Software Foundation, either version 3 of the License, or (at your
+#   option) any later version.
+#
+#   This program is distributed in the hope that it will be useful, but
+#   WITHOUT ANY WARRANTY; without even the implied warranty of
+#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+#   Public License for more details.
+#
+#   You should have received a copy of the GNU General Public License along
+#   with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+#   As a special exception, the respective Autoconf Macro's copyright owner
+#   gives unlimited permission to copy, distribute and modify the configure
+#   scripts that are the output of Autoconf when processing the Macro. You
+#   need not follow the terms of the GNU General Public License when using
+#   or distributing such scripts, even though portions of the text of the
+#   Macro appear in them. The GNU General Public License (GPL) does govern
+#   all other use of the material that constitutes the Autoconf Macro.
+#
+#   This special exception to the GPL applies to versions of the Autoconf
+#   Macro released by the Autoconf Archive. When you make and distribute a
+#   modified version of the Autoconf Macro, you may extend this special
+#   exception to the GPL to apply to your modified version as well.
+
+#serial 3
+
+AC_DEFUN([AX_CHECK_COMPILE_FLAG],
+[AC_PREREQ(2.59)dnl for _AC_LANG_PREFIX
+AS_VAR_PUSHDEF([CACHEVAR],[ax_cv_check_[]_AC_LANG_ABBREV[]flags_$4_$1])dnl
+AC_CACHE_CHECK([whether _AC_LANG compiler accepts $1], CACHEVAR, [
+  ax_check_save_flags=$[]_AC_LANG_PREFIX[]FLAGS
+  _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $4 $1"
+  AC_COMPILE_IFELSE([m4_default([$5],[AC_LANG_PROGRAM()])],
+    [AS_VAR_SET(CACHEVAR,[yes])],
+    [AS_VAR_SET(CACHEVAR,[no])])
+  _AC_LANG_PREFIX[]FLAGS=$ax_check_save_flags])
+AS_IF([test x"AS_VAR_GET(CACHEVAR)" = xyes],
+  [m4_default([$2], :)],
+  [m4_default([$3], :)])
+AS_VAR_POPDEF([CACHEVAR])dnl
+])dnl AX_CHECK_COMPILE_FLAGS