Description:
------------
The PHP_CHECK_GCC_ARG m4 macro in acinclude.m4 does not support flags with equal (=) sign. This affects detection of flags like -std=gnu99 or -fsanitize=address (clang).

I'm experiencing this on a private project, but I've seen at least one open source project getting stuck there: https://github.com/mongodb/mongo-php-driver/blob/c81330c0778a1bd6d314ad3fd7c5d397d5df1dcb/config.m4#L46

The patch is pretty simple, but unfortunately it is backwards incompatible for projects that were (incorrectly) using this macro to check -std=gnu99 support and were working without the check, since the ./configure call may succeed anyway but the flag will not be added. So fixing this issue may cause problems in projects that are using the macro for -std=gnu99 or similar and didn't realize the flag is not added. I'm not sure if that kind of compatibility is supposed to be kept at patch versions or if it would go to the next minor.

Test script:
---------------
PHP_CHECK_GCC_ARG(-std=gnu99, _CFLAGS="$_CFLAGS -std=gnu99")

Expected result:
----------------
$ phpize && ./configure
[...]
checking whether cc supports -std=gnu99... yes
[...]

Actual result:
--------------
$ phpize && ./configure
[...]
checking whether cc supports -std=gnu99... ./configure: line 4221: gnu99+:: command not found
gnu99=yes=gnu99
[...]