Remove the define check for __GNUC_PREREQ.

Our arch.h makes sure it's always defined. Thanks to derf
for the suggestion.

tests/test_opus_api.c

@@ -1168,7 +1168,7 @@ int test_repacketizer_api(void)
  * handling in our codebase, and the lack of thread saftey isn't an
  * issue here. We therefore disable the warning for this function.
  */
-#if defined(__GNUC_PREREQ) && __GNUC_PREREQ(4,6)
+#if __GNUC_PREREQ(4,6)
 /* Save the current warning settings */
 #pragma GCC diagnostic push
 #endif
@@ -1260,8 +1260,7 @@ int test_malloc_fail(void)
 }
 
 #ifdef MALLOC_FAIL
-#if defined(__GNUC_PREREQ) && __GNUC_PREREQ(4,6)
-/* Restore the previous warning settings */
+#if __GNUC_PREREQ(4,6)
 #pragma GCC diagnostic pop /* restore -Wdeprecated-declarations */
 #endif
 #endif