Remove duplicate phpdbg _GNU_SOURCE compile definition (#13585)

The _GNU_SOURCE is defined on few other places when compiling phpdbg
(CPPFLAGS and php_config.h) depending on the system.

Author: petk

sapi/phpdbg/config.m4

@@ -29,7 +29,7 @@ if test "$PHP_PHPDBG" != "no"; then
     AC_DEFINE(PHPDBG_DEBUG, 0, [ ])
   fi
 
-  PHP_PHPDBG_CFLAGS="-D_GNU_SOURCE -DZEND_ENABLE_STATIC_TSRMLS_CACHE=1"
+  PHP_PHPDBG_CFLAGS="-DZEND_ENABLE_STATIC_TSRMLS_CACHE=1"
   PHP_PHPDBG_FILES="phpdbg.c phpdbg_parser.c phpdbg_lexer.c phpdbg_prompt.c phpdbg_help.c phpdbg_break.c phpdbg_print.c phpdbg_bp.c phpdbg_list.c phpdbg_utils.c phpdbg_info.c phpdbg_cmd.c phpdbg_set.c phpdbg_frame.c phpdbg_watch.c phpdbg_btree.c phpdbg_sigsafe.c phpdbg_io.c phpdbg_out.c"
 
   AC_MSG_CHECKING([for phpdbg and readline integration])