ensure vasprintf is available on linux (FSBUILD-160)

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@13358 d0543943-73ff-0310-b7d9-9358b9ac24b2

src/switch_apr.c

@@ -30,6 +30,10 @@
  *
  */
 
+#ifdef __linux__
+#define _GNU_SOURCE
+#endif
+
 #include <switch.h>
 #ifndef WIN32
 #include <switch_private.h>