try to fix clang version detection

pocketfft_hdronly.h

@@ -110,9 +110,15 @@ constexpr bool FORWARD  = true,
 #if defined(__INTEL_COMPILER)
 // do nothing. This is necessary because this compiler also sets __GNUC__.
 #elif defined(__clang__)
+#ifdef __APPLE__
+#if __clang__>=5
+#undef POCKETFFT_NO_VECTORS
+#endif
+#else
 #if __clang_major__>=5
 #undef POCKETFFT_NO_VECTORS
 #endif
+#endif
 #elif defined(__GNUC__)
 #if __GNUC__>=5
 #undef POCKETFFT_NO_VECTORS