Rework r339635 to fix .depend.tables.h handling.

Avoid touching the tables.h file unless it has changed to avoid unneeded
rebuilds.

Also revert r350301's explicit dependencies.

Reviewed by:	emaste
MFC after:	2 weeks
X-MFC-With:	r339635 (kevans request)
PR:		238828
Sponsored by:	DellEMC
Differential Revision:	https://reviews.freebsd.org/D21295

lib/libsysdecode/Makefile

@@ -107,7 +107,7 @@ MLINKS+=sysdecode_mask.3 sysdecode_accessmode.3 \
 	sysdecode_mask.3 sysdecode_wait4_options.3 \
 	sysdecode_mask.3 sysdecode_wait6_options.3
 
-CLEANFILES= ioctl.c ioctl.c.tmp tables.h tables.h.tmp
+CLEANFILES= ioctl.c ioctl.c.tmp tables.h
 
 .if defined(COMPAT_32BIT)
 CPP+=	-m32
@@ -123,11 +123,8 @@ CFLAGS.gcc.ioctl.c+= -Wno-redundant-decls
 CFLAGS.gcc+=	${CFLAGS.gcc.${.IMPSRC}}
 
 DEPENDOBJS+=	tables.h
-incdir=${SYSROOT:U${DESTDIR}}${INCLUDEDIR}
-tables.h: mktables ${incdir}/netinet/in.h ${incdir}/netinet/tcp.h \
-    ${incdir}/netinet6/in6.h
-	sh ${.CURDIR}/mktables ${incdir} ${.TARGET}.tmp && \
-	    mv -f ${.TARGET}.tmp ${.TARGET}
+tables.h: mktables
+	sh ${.CURDIR}/mktables ${SYSROOT:U${DESTDIR}}${INCLUDEDIR} ${.TARGET}
 
 # mkioctls runs find(1) for headers so needs to rebuild every time.  This used
 # to be a hack only done in buildworld.

lib/libsysdecode/mktables

@@ -43,7 +43,8 @@ fi
 include_dir=$1
 if [ -n "$2" ]; then
 	output_file="$2"
-	exec > "$output_file"
+	output_tmp=$(mktemp -u)
+	exec > "$output_tmp"
 fi
 
 all_headers=
@@ -167,9 +168,17 @@ fi
 
 # Generate a .depend file for our output file
 if [ -n "$output_file" ]; then
-	echo "$output_file: \\" > ".depend.$output_file"
-	echo "$all_headers" | tr ' ' '\n' | sort -u |
-	    sed -e "s,^,	$include_dir/," -e 's,$, \\,' >> \
-	    ".depend.$output_file"
-	echo >> ".depend.$output_file"
+	depend_tmp=$(mktemp -u)
+	{
+		echo "$output_file: \\"
+		echo "$all_headers" | tr ' ' '\n' | sort -u |
+		    sed -e "s,^,	$include_dir/," -e 's,$, \\,'
+		echo
+	} > "$depend_tmp"
+	if cmp -s "$output_tmp" "$output_file"; then
+		rm -f "$output_tmp" "$depend_tmp"
+	else
+		mv -f "$depend_tmp" ".depend.${output_file}"
+		mv -f "$output_tmp" "$output_file"
+	fi
 fi