fix undefined vasprintf

Fix compile issue with recent oneapi+icx compiler

include/chameleon.h

@@ -28,6 +28,7 @@
  * CHAMELEON types and constants
  */
 #include "chameleon/config.h"
+#define _GNU_SOURCE 1
 #include <stdio.h>
 #include "chameleon/constants.h"
 #include "chameleon/types.h"