Fix GH-7978: sockets extension compilation errors

We fix the `ucred` detection when custom `CFLAGS` are in use.

Closes GH-7981.

Author: devnexen

NEWS

@@ -22,6 +22,7 @@ PHP                                                                        NEWS
 
 - Sockets:
   . Fixed ext/sockets build on Haiku. (David Carlier)
+  . Fixed bug GH-7978 (sockets extension compilation errors). (David Carlier)
 
 - Standard:
   . Fixed bug GH-7875 (mails are sent even if failure to log throws exception).

ext/sockets/config.m4

@@ -67,6 +67,9 @@ if test "$PHP_SOCKETS" != "no"; then
   AC_CACHE_CHECK([if ancillary credentials uses ucred],[ac_cv_ucred],
   [
     AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
 #include <sys/socket.h>
   ]], [[struct ucred u = {.gid = 0};]])],
       [ac_cv_ucred=yes], [ac_cv_ucred=no])