gbm: Define _DEFAULT_SOURCE to avoid warning

glibc 2.19 introduced _DEFUAULT_SOURCE as a replacement for _BSD_SOURCE,
and deprecates _BSD_SOURCE with an annoying warning.  Defining both is
how you're supposed to transition so let's do that.  It gets rid of the
warning and we can figure out when/if we can drop _BSD_SOURCE later.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>

src/gbm/main/gbm.c

@@ -26,6 +26,7 @@
  */
 
 #define _BSD_SOURCE
+#define _DEFAULT_SOURCE
 
 #include <stddef.h>
 #include <stdio.h>