[config] support __builtin_expect LIKELY/UNLIKELY branch optimizations

add EXPECT,LIKELY,UNLIKELY macros to feature.h as in the linux kernel and perl5
probe for __builtin_expect() support in a new auto::expect configure step

Usage: if (UNLIKELY(var == 0)) fail();

Add emacs mode:c support for test_c.in files, and relax the coda test for this.
Should be used in all test_c.in files.

config/gen/config_h/feature_h.in

@@ -231,6 +231,28 @@ END_PRINT
     }
 }
 
+/* from config/auto/expect */
+END_PRINT
+if (@has_builtin_expect@) {
+    print OUT <<'END_PRINT';
+#define PARROT_HAS_BUILTIN_EXPECT 1
+END_PRINT
+}
+else {
+    print OUT <<'END_PRINT';
+#undef PARROT_HAS_BUILTIN_EXPECT
+END_PRINT
+}
+print OUT <<'END_PRINT';
+#ifdef PARROT_HAS_BUILTIN_EXPECT
+#  define EXPECT(expr,val)                  __builtin_expect(expr,val)
+#else
+#  define EXPECT(expr,val)                  (expr)
+#endif
+#define LIKELY(cond)                        EXPECT((cond),1)
+#define UNLIKELY(cond)                      EXPECT((cond),0)
+END_PRINT
+
 print OUT <<'END_PRINT';
 
 /* from config/auto/expect */