allow external lua

Created by: davydden

WIP, but internal lua seems to build for me on Ubuntu 16.04, so this part should be fine.

p.s. adding support of external libs in makefile-only packages is a grand royal PITA 😄 I have not tested it mostly because lua i have around is a static lib with, presumably, -fPIC flags. So it should be fine to use it in shared libraries, but I seriously don't know what else/how needs to be changed to support this usage case.