Makefile: cleanup usage of libdir and install user units

Hi,

I need those changed to have envoy compiled on 64-bit Fedora 21.

  Actually, /usr/lib/systemd is an API-like path, there is no variables in /lib/ part - hardcode it.
  On the other hand, PAM's libdir could be lib or lib64. Use pkg-config to determine that.
  Lastly, install system-wide user units, too.