Honor LUAC passed by the user.

As we use AC_PATH_PROG, if you want to force LUAC by specifying it on your command line you need to give a full path+exec_name. Which is a bit un friendly. So work-around this behaviour.
(cherry picked from commit 8fb9110b)
Signed-off-by: Christophe Mutricy <xtophe@videolan.org>

configure.ac

@@ -1631,7 +1631,7 @@ then
       AC_MSG_ERROR([Could not find lua. Lua is needed for some interfaces (rc, telnet, http) as well as many other custom scripts. Use --disable-lua to ignore this error.])
   fi
   AC_ARG_VAR([LUAC], [LUA byte compiler])
-  AC_PATH_PROG(LUAC, luac, [false])
+  AC_PATH_PROGS(LUAC,[${LUAC} luac], [false])
   AS_IF([test "${ac_cv_path_LUAC}" = "false"], [
     AC_MSG_ERROR([Could not find the LUA byte compiler.])
   ])