ax_gcc_func_attribute: Revise the detection of unknown attributes

ax_gcc_func_attribute: Revise the detection of unknown attributes

GCC outputs a warning when Wstrict-prototypes is on, in such case the
attribute detection always fails even if the attribute is actually
supported. This change checks for the "-Wattributes" warning in
conftest.err instead of the existence of the file.

Bug-Ubuntu: https://bugs.launchpad.net/ubuntu/+source/php8.1/+bug/1882279
Forwarded: https://github.com/php/php-src/pull/8483
Origin: http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=commitdiff;h=df0894ad1a8195df67a52108b931e07d708cec9a
Last-Update: 2022-05-02

Gbp-Pq: Name 0039-Update-gcc-func-attr-macro.patch