Guard inclusion of emmintrin.h

Guard inclusion of emmintrin.h with "#ifdef __SSE2__". The gcc version
of this header, xmmintrin.h, and mmintrin.h check that the appropriate
defines are set before defining anything (are internally guarded).
However the clang versions of these includes are not internally guarded.
As a result of this, externally guard the inclusion of these headers.
diff --git a/src/smooth/ftgrays.c b/src/smooth/ftgrays.c
index 7158cd2..5e04ff4 100644
--- a/src/smooth/ftgrays.c
+++ b/src/smooth/ftgrays.c
@@ -1009,7 +1009,9 @@
+#ifdef __SSE2__
 #include <emmintrin.h>
   static void
   gray_render_conic( RAS_ARG_ const FT_Vector*  control,