Force proper libdir for LuaJIT

I have found that the LuaJIT spec file doesn't build on my CentOS 6 x86_64 machines, because the "make install" step copies the libs to /usr/lib (in the buildroot) as opposed to /usr/lib64

I've patched the make lines to include the libdir macro so that the libraries are placed in the buildroot in the appropriate architecture-specific directory