diff --git a/sapi/phpdbg/config.m4 b/sapi/phpdbg/config.m4
index a6b3a28f2d36d..4a7b1810e9075 100644
--- a/sapi/phpdbg/config.m4
+++ b/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])