Add _GNU_SOURCE definition unconditionally (#14651)

Since Autoconf >= 2.70 the _GNU_SOURCE and some other extensions are
defined unconditionally regardless whether the __EXTENSIONS__ can be
safely defined. Some obsolete Solaris systems once had issues with
compiling a default set of standard system headers when __EXTENSIONS__
was defined. Autoconf 2.69 and earlier checked based on that and defined
the _GNU_SOURCE conditionally. This condition is not relevant on current
systems anymore.

Author: petk

configure.ac

@@ -259,12 +259,10 @@ else
   AC_MSG_RESULT(no)
 fi
 
-dnl The effect of _GNU_SOURCE defined in config.h depeds on includes order
-if test "$ac_cv_safe_to_define___extensions__" = yes ; then
-  AC_MSG_CHECKING(whether to use -D_GNU_SOURCE cflag)
-  CPPFLAGS="$CPPFLAGS -D_GNU_SOURCE"
-  AC_MSG_RESULT(yes)
-fi
+dnl Add _GNU_SOURCE compile definition because the php_config.h with definitions
+dnl by AC_USE_SYSTEM_EXTENSIONS might be included after the system headers which
+dnl require extensions to C and POSIX.
+AS_VAR_APPEND([CPPFLAGS], [" -D_GNU_SOURCE"])
 
 dnl Include Zend configurations.
 dnl ----------------------------------------------------------------------------