Allow easier building with different Lua libraries.

  This involves changing the way we determine the package
 name, and specify the version of Lua itself:

        make LUA_VERSION=5.1
        make LUA_VERSION=5.2

  This updates #181.
  Updated to work with Lua 5.2

  I'd accidentally used some older code which worked under Lua
 5.1, but not 5.2:

> What used to be called lua_objlen in 5.1 was been renamed to lua_rawlen, with the only difference in behaviour being that lua_rawlen no longer calculates the length of a number by taking the string representation of it; it just returns zero.