use AC_USE_SYSTEM_EXTENSIONS instead of hard coding _GNU_SOURCE

Instead of just enabling linux specific libc extension, do it
generically on all platforms.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

configure.ac

@@ -34,6 +34,7 @@ AM_INIT_AUTOMAKE([foreign dist-xz])
 AM_MAINTAINER_MODE
 AC_CONFIG_HEADERS([config.h])
 AC_SYS_LARGEFILE
+AC_USE_SYSTEM_EXTENSIONS
 
 # Require xorg-macros: XORG_DEFAULT_OPTIONS
 m4_ifndef([XORG_MACROS_VERSION],

src/qxl_driver.c

@@ -32,10 +32,6 @@
 #include "config.h"
 #endif
 
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE
-#endif
-
 #include <unistd.h>
 #include <string.h>
 #include <stdio.h>

src/qxl_kms.c

@@ -24,10 +24,6 @@
 #include <config.h>
 #endif
 
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE
-#endif
-
 #include <stdio.h>
 
 #ifdef XF86DRM_MODE