Use AX_CHECK_COMPILE_FLAG macro to check for -fvisibility=hidden

[orlitzky]

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

[bukka]

I'm not sure if we can actually use it due to license of AX_CHECK_COMPILE_FLAG:

http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=blob_plain;f=m4/ax_check_compile_flag.m4

It looks to me that it is GPL code but I'm not sure either what consequences it could have... :)

[orlitzky]

Good catch. There are a number of files in the repository that have a non-PHP license block at the top, indicating that their licenses differ from the default PHP-3.01 license. In particular, there are other autoconf macros with a GPL license like AX_CHECK_COMPILE_FLAG has; for example, TSRM/m4/gethostbyname.m4.

It would be better to ship ax_check_compile_flag.m4 separately. But where should I put that file? Should I create a top-level m4 directory, then then use something like m4_include to load it?

[krakjoe]

Merged 9e18b6e

Thanks :)

[krakjoe]

This breaks some stuff ... pecl extensions can't compile because they are looking for the m4.

I'm on it ...

[krakjoe]

Fixed.