test_opus_decode: Fix build on GCC 3.4 to 4.5.x

Broken by b0949f11, reported by ko-zu on GitHub.

tests/test_opus_decode.c

@@ -135,12 +135,12 @@ int test_decoder_code0(int no_fuzz)
          outbuf[0]=32749;
          out_samples = opus_decode(dec[t], packet, 0, outbuf, 0, fec);
          if(out_samples>0)test_failed();
-#if defined(__GNUC__) && OPUS_GNUC_PREREQ(3, 4)
+#if !defined(OPUS_BUILD) && (OPUS_GNUC_PREREQ(4, 6) || (defined(__clang_major__) && __clang_major__ >= 3))
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wnonnull"
 #endif
          out_samples = opus_decode(dec[t], packet, 0, 0, 0, fec);
-#if defined(__GNUC__) && OPUS_GNUC_PREREQ(3, 4)
+#if !defined(OPUS_BUILD) && (OPUS_GNUC_PREREQ(4, 6) || (defined(__clang_major__) && __clang_major__ >= 3))
 #pragma GCC diagnostic pop
 #endif
          if(out_samples>0)test_failed();