Switch to in-tree Lua 5.3.

I suggest this for a couple of reasons:

- It allows you to tightly enforce which Lua versions you're compiling against, which is important for consistencies sake.
- It allows you to make customizations to the Lua implementation if you so desire.  For example, Garry's Mod modified their Lua implementation to allow scripts to use C-style syntax.
- It allows you to support multiple Lua versions easier.  You could keep complete compatibility for scripts written against the older Lua versions while supporting new versions of Lua at the same time.  I don't know what the Lua versioning story is on other platforms offhand, but at least on my mac Homebrew won't even allow me to have 5.1 and 5.2 installed at the same time.

As for why Lua 5.3, it's not officially out yet, but has some really nice new features that might make it easier to script Doom, like real integer support and built-in bitshifting.

I've verified that d2k still compiles with my changes.  Unfortunately, d2k segfaults on this machine, with and without my changes, so I can't say if it broke anything.