Optional lua for debugging

See the commit for more details.

I tested compilation with LUA=1 and without, but tests on more systems would be good. Right now the Makefile only includes support for the way things are set up on arch linux, since I have no damn clue where the lua 5.1 stuff is on other systems.

EDIT: I had planned to use CMake to get this to run on all systems easily, but it turns out the CMake lua51 include file is broken. So yeah, gonna have to fix up the Makefile with all the platforms.