Add back required _XOPEN_SOURCE define

This got lost somewhere between 2.30 and now, but it is needed
for isnan(). See bug 664354, reported by Tim Mooney.

glib/tests/strfuncs.c

@@ -21,6 +21,7 @@
 
 #define GLIB_DISABLE_DEPRECATION_WARNINGS
 
+#define _XOPEN_SOURCE 600
 #include <ctype.h>
 #include <errno.h>
 #include <locale.h>