Added option LUA_NOBUILTIN

This option allows external code to avoid the use of gcc builtin macro
'__builtin_expect' in the Lua API.

luaconf.h

@@ -663,11 +663,13 @@
 
 /*
 ** macros to improve jump prediction, used mostly for error handling
-** and debug facilities.
+** and debug facilities. (Some macros in the Lua API use these macros.
+** Define LUA_NOBUILTIN if you do not want '__builtin_expect' in your
+** code.)
 */
 #if !defined(luai_likely)
 
-#if defined(__GNUC__)
+#if defined(__GNUC__) && !defined(LUA_NOBUILTIN)
 #define luai_likely(x)		(__builtin_expect(((x) != 0), 1))
 #define luai_unlikely(x)	(__builtin_expect(((x) != 0), 0))
 #else