Only use Clang-specific pragma if compiler is Clang

Otherwise the compiler will probably give us a warning that these
pragmas are unknown. By default that warning is disabled with our
own build system, but we also want to be able to link to our library
without incorporating the entire build system too.

opm/core/simulator/initState_impl.hpp

@@ -39,9 +39,10 @@ namespace Opm
 
     namespace
     {
-
+#ifdef __clang__
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wunneeded-internal-declaration"
+#endif /* __clang__ */
         // Find the cells that are below and above a depth.
         // TODO: add 'anitialiasing', obtaining a more precise split
         //       by f. ex. subdividing cells cut by the split depths.
@@ -63,7 +64,9 @@ namespace Opm
                 }
             }
         }
+#ifdef __clang__
 #pragma clang diagnostic pop
+#endif /* __clang__ */
 
         enum WaterInit { WaterBelow, WaterAbove };