Some setups need _GNU_SOURCE to make LLONG_MAX available (thanks, Ozkan!).

Fixes Bugzilla #2721.

src/test/SDL_test_fuzzer.c

@@ -34,6 +34,7 @@
 #define UINT32_MAX  ~(Uint32)0
 #define UINT64_MAX  ~(Uint64)0
 #else
+#define _GNU_SOURCE
 #include <stdint.h>
 #endif
 #include <stdio.h>