Fix pixmap compilation with clang

Clang does not allow a variable to be used for the "K" register, even if
all uses of the inlined function use a constant value.

The current version of pixmap doesn't use the inline function. It uses a
macro regardless of the optimization level. There should be no
performance overhead from using the macro.

xorg/X11R7.6/pixman-0.30.0.patch

@@ -0,0 +1,11 @@
+--- a/pixman/pixman-mmx.c
++++ b/pixman/pixman-mmx.c
+@@ -89,7 +89,7 @@ _mm_mulhi_pu16 (__m64 __A, __m64 __B)
+     return __A;
+ }
+ 
+-#  ifdef __OPTIMIZE__
++#  if defined(__OPTIMIZE__) && !defined(__clang__)
+ extern __inline __m64 __attribute__((__gnu_inline__, __always_inline__, __artificial__))
+ _mm_shuffle_pi16 (__m64 __A, int8_t const __N)
+ {