Disable MMX when Clang is being used.

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.