Rely on pkg-config for libsystemd-daemon location

Hardcoded `.so` location doesn't work everywhere - for example, on i686 systems there's no `/libs64/` at all. See https://github.com/TooTallNate/node-gyp/wiki/%22binding.gyp%22-files-out-in-the-wild for solutions people use. Since `pkg-config` config files for systemd are maintained by systemd developers as evident from `systemd` man pages, I think `pkg-config` is the best solution.