Instead of including the `luabind.hpp` header that transitively includes
basically everything else, we now only include the appropriate header
for luabind's open function.
It is important that this function is declared in the header but the
definition comes from the luabind shared object (library), such that we
can detect linker errors, too.
By only including this header, we also no longer transitively include
the header for the `luaL_newstate` function, with we have to add
manually.
With these changes, detecting, compiling, linking and checking for
lua/luabind with cmake now works instantly, instead of the 3-4
seconds as it was before! Yay, progress!
