opal_stdint.h: fix "#pragma GCC" warnings

This was more complicated than I would like, but it's just an
unfortunate GCC/clang difference.  I don't have access to all the C
compilers out there, so this may still have problems with other
compilers that implement some form of `#pragma GCC diagnostic` support
but don't actually behave the same as some versions of GCC.

fixes #323

opal/include/opal_stdint.h

@@ -147,7 +147,14 @@ typedef uint128_t opal_uint128_t;
 
 /* suppress warning about __int128 type */
 #pragma GCC diagnostic push
+/* Clang won't quietly accept "-pedantic", but GCC versions older than ~4.8
+ * won't quietly accept "-Wpedanic".  The whole "#pragma GCC diagnostic ..."
+ * facility only was added to GCC as of version 4.6. */
+#if defined(__clang__)
 #pragma GCC diagnostic ignored "-Wpedantic"
+#else
+#pragma GCC diagnostic ignored "-pedantic"
+#endif
 typedef __int128 opal_int128_t;
 typedef unsigned __int128 opal_uint128_t;
 #pragma GCC diagnostic pop