Make sure iwMMXt is only detected on ARM

iwMMXt is incorrectly detected on x86 and amd64. This happens because
the test uses standard _mm_* intrinsic functions which it compiles with
-march=iwmmxt, but when the user has set CFLAGS=-march=k8 for instance,
no error is generated from -march=iwmmxt, even though it's not a valid
flag on x86/amd64. Passing CFLAGS=-march=native does not override the
-march=iwmmxt flag though, which is why it wasn't noticed before.

So, just #error out in the test if the __arm__ preprocessor directive
isn't defined.

Fixes https://bugs.gentoo.org/show_bug.cgi?id=385179



Signed-off-by: Matt Turner <mattst88@gmail.com>

configure.ac

@@ -552,6 +552,9 @@ AC_MSG_CHECKING(whether to use ARM IWMMXT intrinsics)
 xserver_save_CFLAGS=$CFLAGS
 CFLAGS="$IWMMXT_CFLAGS $CFLAGS"
 AC_COMPILE_IFELSE([
+#ifndef __arm__
+#error "IWMMXT is only available on ARM"
+#endif
 #if defined(__GNUC__) && (__GNUC__ < 4 || (__GNUC__ == 3 && __GNUC_MINOR__ < 6))
 #error "Need GCC >= 4.6 for IWMMXT intrinsics"
 #endif