ensure vasprintf is available on linux (FSBUILD-160)

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@13361 d0543943-73ff-0310-b7d9-9358b9ac24b2

src/switch_apr.c

@@ -31,8 +31,10 @@
  */
 
 #ifdef __linux__
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
 #endif
+#endif
 
 #include <switch.h>
 #ifndef WIN32