Update gcc-lua & gcc-lua-cdecl to latest releases
Shrink the Makefile to the strict minimum.

Some of it is to adapt to the new releases, but I might have killed some
useful stuff for certain setups.
Update the scripts.

This is basically the example from
http://colberg.org/gcc-lua-cdecl/ffi-cdecl.html
with a few minor cosmetic tweaks.