configure: Use the full path to luac to support our contrib system.

This fixes the build on Mac OS X. This is probably not the best way
for non-contrib dependant build, but I don't have any better idea.
Feel free to make it works differently.

configure.ac

@@ -1620,7 +1620,7 @@ then
   else
       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_CHECK_PROGS(LUAC, luac)
+  AC_PATH_PROG(LUAC, luac)
 fi
 AM_CONDITIONAL(BUILD_LUA, [test "${have_lua}" = "yes"])