Issue with undefined _GNU_SOURCE

Description

Original report: https://bugzilla.redhat.com/2271925

$ gcc -c -g -O2  -I/usr/include/php -I/usr/include/php/main -I/usr/include/php/TSRM -I/usr/include/php/Zend -I/usr/include/php/ext -I/usr/include/php/ext/date/lib conftest.c >&5
In file included from /usr/include/php/Zend/zend.h:418,
                 from /usr/include/php/main/php.h:32,
                 from conftest.c:63:
/usr/include/php/Zend/zend_operators.h: In function 'zend_memrchr':
/usr/include/php/Zend/zend_operators.h:207:29: error: implicit declaration of function 'memrchr'; did you mean 'memchr'? [-Wimplicit-function-declaration]
  207 |         return (const void*)memrchr(s, c, n);
      |                             ^~~~~~~
      |                             memchr
/usr/include/php/Zend/zend_operators.h:207:16: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
  207 |         return (const void*)memrchr(s, c, n);
      |                ^
configure:3238: $? = 1



configure: failed program was:
| /* confdefs.h */
| #define PACKAGE_NAME "test"
| #define PACKAGE_TARNAME "test"
| #define PACKAGE_VERSION "1.0.0-1"
| #define PACKAGE_STRING "test 1.0.0-1"
| #define PACKAGE_BUGREPORT ""
| #define PACKAGE_URL ""
| #define HAVE_STDIO_H 1
| #define HAVE_STDLIB_H 1
| #define HAVE_STRING_H 1
| #define HAVE_INTTYPES_H 1
| #define HAVE_STDINT_H 1
| #define HAVE_STRINGS_H 1
| #define HAVE_SYS_STAT_H 1
| #define HAVE_SYS_TYPES_H 1
| #define HAVE_UNISTD_H 1
| #define STDC_HEADERS 1
| /* end confdefs.h.  */
| #include <stddef.h>
| #ifdef HAVE_STDIO_H
| # include <stdio.h>
| #endif
| #ifdef HAVE_STDLIB_H
| # include <stdlib.h>
| #endif
| #ifdef HAVE_STRING_H
| # include <string.h>
| #endif
| #ifdef HAVE_INTTYPES_H
| # include <inttypes.h>
| #endif
| #ifdef HAVE_STDINT_H
| # include <stdint.h>
| #endif
| #ifdef HAVE_STRINGS_H
| # include <strings.h>
| #endif
| #ifdef HAVE_SYS_TYPES_H
| # include <sys/types.h>
| #endif
| #ifdef HAVE_SYS_STAT_H
| # include <sys/stat.h>
| #endif
| #ifdef HAVE_UNISTD_H
| # include <unistd.h>
| #endif
| #include <php.h>
configure:3238: result: no
configure:3243: PHP haders not found or test compile failed

Notice: related to GCC 14 where -Wimplicit-function-declaration now raises an error

A workaround is to use -D_GNU_SOURCE

I try to dig where it is missing (zend_operators.h ?), but cannot find a proper place to add it (we already have tons of places where it is defined)

Would it be simpler to define it globally and earlier?

PHP Version

PHP 8.3

Operating System

Fedora 40 with GCC 14

Start with Zend/zend_operators.h and the configure compile check that includes php.h, then reproduce the failure with GCC 14 on Fedora 40. Trace where _GNU_SOURCE is currently defined and determine the appropriate scope for fixing the missing memrchr declaration. Done means the reported configuration test compiles without the workaround.