Disable MMX when Clang is being used.

There are several issues with the Clang compiler and pixman-mmx.c:

- When not optimizing, it doesn't seem to recognize that an argument
  to an __always_inline__ function is compile-time constant. This
  results in this error being produced:

      fatal error: error in backend: Invalid operand for inline asm
              constraint 'K'!

- This inline assembly:

      asm ("pmulhuw %1, %0\n\t"
          : "+y" (__A)
          : "y" (__B)
      );

  results in

      fatal error: error in backend: Unsupported asm: input constraint
              with a matching output constraint of incompatible type!

So disable MMX when the compiler is Clang.

1 files changed, 3 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index f39f4373..cca4d16e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -294,6 +294,9 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
 #if defined(__GNUC__) && (__GNUC__ < 3 || (__GNUC__ == 3 && __GNUC_MINOR__ < 4))
 #error "Need GCC >= 3.4 for MMX intrinsics"
 #endif
+#if defined(__clang__)
+#error "clang chokes on the inline assembly in pixman-mmx.c"
+#endif
 #include <mmintrin.h>
 int main () {
     __m64 v = _mm_cvtsi32_si64 (1);