build: Use non-deprecated feature test macros

_SVID_SOURCE has been deprecated in newer versions of glibc breaking
-WError; the recommended replacement of _DEFAULT_SOURCE is fairly
new, so switch to _XOPEN_SOURCE instead.

src/core/main.c

@@ -42,7 +42,7 @@
  */
 
 #define _GNU_SOURCE
-#define _SVID_SOURCE /* for putenv() and some signal-related functions */
+#define _XOPEN_SOURCE /* for putenv() and some signal-related functions */
 
 #include <config.h>
 #include <meta/main.h>

src/core/window-props.c

@@ -35,7 +35,7 @@
  */
 
 #define _GNU_SOURCE
-#define _SVID_SOURCE /* for gethostname() */
+#define _XOPEN_SOURCE 500 /* for gethostname() */
 
 #include <config.h>
 #include "window-props.h"