Isolate #pragma clang to __clang__ compilations.

It's very irritating that Visual Studio produces a warning for unrecognized
#pragmas, which we then merrily turn into an error -- #pragma is inherently
compiler-specific!

indra/llcommon/tests/llerror_test.cpp

@@ -38,7 +38,9 @@
 
 namespace
 {
+#ifdef __clang__
 #   pragma clang diagnostic ignored "-Wunused-function"
+#endif
 	void test_that_error_h_includes_enough_things_to_compile_a_message()
 	{
 		LL_INFOS() << "!" << LL_ENDL;