[glib] use proper feature test macros for isnan

[Matthias Clasen <matthiasc src gnome org>]

commit 7568bb66f996bdf94da6963399333236f73adb1b
Author: Matthias Clasen <mclasen redhat com>
Date:   Sat Jul 10 20:00:49 2010 -0400

    use proper feature test macros for isnan
    
    Problem pointed out in bug 581354

 glib/tests/strfuncs.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/glib/tests/strfuncs.c b/glib/tests/strfuncs.c
index 2ddf926..648f370 100644
--- a/glib/tests/strfuncs.c
+++ b/glib/tests/strfuncs.c
@@ -19,6 +19,7 @@
  * if advised of the possibility of such damage.
  */
 
+#define _XOPEN_SOURCE
 #include <ctype.h>
 #include <errno.h>
 #include <locale.h>