stb_sprintf: GCC compilation fix

Fixes issue #1010.

stb_sprintf.h

@@ -154,8 +154,8 @@ PERFORMANCE vs MSVC 2008 32-/64-bit (GCC is even slower than MSVC):
    #endif
   #endif
  #endif
-#elif __GNUC__ >= 5 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 8)
- #if __SANITIZE_ADDRESS__
+#elif defined(__GNUC__) && (__GNUC__ >= 5 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 8))
+ #if defined(__SANITIZE_ADDRESS__) && __SANITIZE_ADDRESS__
   #define STBSP__ASAN __attribute__((__no_sanitize_address__))
  #endif
 #endif