use 'log2' when available

lmathlib.c

@@ -1,5 +1,5 @@
 /*
-** $Id: lmathlib.c,v 1.113 2014/11/07 11:31:58 roberto Exp roberto $
+** $Id: lmathlib.c,v 1.114 2014/12/27 20:32:26 roberto Exp roberto $
 ** Standard mathematical library
 ** See Copyright Notice in lua.h
 */
@@ -183,6 +183,9 @@ static int math_log (lua_State *L) {
     res = l_mathop(log)(x);
   else {
     lua_Number base = luaL_checknumber(L, 2);
+#if !defined(LUA_USE_C89)
+    if (base == 2.0) res = l_mathop(log2)(x); else
+#endif
     if (base == 10.0) res = l_mathop(log10)(x);
     else res = l_mathop(log)(x)/l_mathop(log)(base);
   }