configure.ac was introduced in 2001 with automake-1.15 and autoconf-2.50
to replace the file named configure.in.
Autotools is preparing to remove configure.in in Automake 2.0.
All new software should be using configure.ac.
This also fixes Bug #69770 where extensions are creating configure.in

Signed-off-by: Brian Evans <grknight@gentoo.org>

* pull-request/2124:
  Use AX_CHECK_COMPILE_FLAG macro to check for -fvisibility=hidden

* pull-request/2163:
  import explicit_bzero + strlc* functions update since 1999 algorithms have changed and register k/w not necessary anymore.

If this does not break the Unix system somehow, I'll be amazed. This should get most of it out, apologies for any errors this may cause on non-Windows ends which I cannot test atm.

since 1999 algorithms have changed and register k/w
not necessary anymore.

The existing check for -fvisibility=hidden support came from a time
when only GCC had it. The test for it used a regular expression to
parse the GCC major version from the output of `$CC --version`, and
would look for version 4 or greater.

The regular expression used to accomplish this is doomed, however,
since GCC can be built with a custom version string
(--with-pkgversion). Moreover, the $CC variable can be set to
something that confuses it but is otherwise valid. For example, it
would fail with CC=x86_64-pc-linux-gnu-gcc.

This commit fixes two aspects of the feature test. First, it no longer
limits the test to GCC. At least clang now supports the flag, and can
make use of it when GCC is its backend. Second, support for the flag
is tested directly, by attempting to pass it to the compiler, rather
than by parsing its version string. The latter is accomplished with a
new macro, AX_CHECK_COMPILE_FLAG, taken from the autoconf archive.
The new macro has been added to acinclude.m4, and the test stanza in
configure.in has been replaced with a single call to the new macro.

Note that the new macro calls AC_PREREQ(2.64) -- a more stringent
requirement than the existing AC_PREREQ(2.59) in configure.in. The
difference represents about six years of autoconf releases, from v2.59
in December of 2003 to v2.64 in July of 2009.

This problem was noticed by Brian Evans, who also suggested the fix.

PHP-Bug: 73062