fix clang detection

pocketfft_hdronly.h

@@ -105,7 +105,7 @@ constexpr bool FORWARD  = true,
 #if defined(__INTEL_COMPILER)
 // do nothing. This is necessary because this compiler also sets __GNUC__.
 #elif defined(__clang__)
-#if __clang__>=5
+#if __clang_major__>=5
 #undef POCKETFFT_NO_VECTORS
 #endif
 #elif defined(__GNUC__)