Allow for use of Lua 5.5

Allow for use of Lua 5.5

In Lua 5.5, the signature for lua_newstate changes
(there is now an additional "seed" argument),
and the mechanism for adjusting garbage collection also changes.

I don't know whether the seed of 0 is ideal when using lua_newstate;
probably, using a random seed would be better. This is a minimal patch
that gets back to a working build.