Lua 5.3 support

[daurnimator]

5.3 is not mentioned in docs and not allowed by the rockspec.

However there does seem to be a section for it in the Makefile.

[peersuasive]

This is available with the last committed Linux/Makefile:

make CONFIG=Release LUA_V=53

I haven't maintained the rockspec for a while... I'll think about it for the next release.