Remove AM_PROG_AR.

With AM_PROG_AR may report "error: possibly underfined macro: AM_PROG_AR" (mingw, autoreconf-2.68).
Without AM_PROG_AR may report warning "archiver requires 'AM_PROG_AR' in 'configure.ac'" (cygwin, automake-1.12).
They do not affect the tests and install

configure.ac

@@ -46,7 +46,6 @@ AC_PROG_CC
 AC_PROG_CXX
 AC_LANG([C++])
 ACX_USE_SYSTEM_EXTENSIONS
-AM_PROG_AR
 AM_CONDITIONAL(GCC, test "$GCC" = yes) # let the Makefile know if we're gcc
 
 # test_util.cc takes forever to compile with GCC and optimization turned on.