meson.build: Improve F16C detection on Visual Studio

The __builtin_cpu...() intrisics are strictly for GCC/CLang-based compilers, so
don't use them in the checking code on Visual Studio.  The test code will still
compile without this change, but will certainly fail if we want it to link.

meson.build

@@ -707,8 +707,10 @@ int main () {
   __m128i i = _mm_cvtps_ph (s, 0);
   _mm_storel_epi64 ((__m128i*)h, i);
 
+#if defined (__GNUC__) || defined (__clang__)
   __builtin_cpu_init ();
   __builtin_cpu_supports ("f16c");
+#endif
 
     return 0;
 }'''