Add predicate likelihood macros.

Define the couple of standard macros that we can use to guide gcc
optimisations of which code path will be most likely taken.

src/cairo-compiler-private.h

@@ -139,6 +139,23 @@
 #define cairo_const
 #endif
 
+#if defined(__GNUC__) && (__GNUC__ > 2) && defined(__OPTIMIZE__)
+#define _CAIRO_BOOLEAN_EXPR(expr)                   \
+ __extension__ ({                               \
+   int _cairo_boolean_var_;                         \
+   if (expr)                                    \
+      _cairo_boolean_var_ = 1;                      \
+   else                                         \
+      _cairo_boolean_var_ = 0;                      \
+   _cairo_boolean_var_;                             \
+})
+#define _cairo_likely(expr) (__builtin_expect (_CAIRO_BOOLEAN_EXPR(expr), 1))
+#define _cairo_unlikely(expr) (__builtin_expect (_CAIRO_BOOLEAN_EXPR(expr), 0))
+#else
+#define _cairo_likely(expr) (expr)
+#define _cairo_unlikely(expr) (expr)
+#endif
+
 #ifndef __GNUC__
 #undef __attribute__
 #define __attribute__(x)