Install lua files under the confiugred --prefix rather than use the
pkg-config's variables LUA_INSTALL_[CL]MOD.

Users will likely want user --prefix while packagers will use DESTDIR.
Set the default to $datadir/lua/$LUA_VERSION for arch independent
lua modules and $libdir/lua/$LUA_VERSION for arch dependant .so module.

This should work for most distros. If it does not, then packagers
can still do:

  make install lualibdir=$(pkg-config lua --variable=INSTALL_CMOD) ...

This fixes #169
Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Acked-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>