Fix Lua includes on OS X

WARNING: This will probably break non-Apple builds.

Macports on OS X places the Lua header files directly in /opt/local/include, instead of creating a "lua" containing folder.  I would recommend modifying the CMAKE module to add "<path to includes>/lua" as a header search path for any platform that places the Lua headers in their own folder.  Alternatively, we could add an #ifdef __APPLE__ to the code where the Lua headers are included.