Add systemd system sleep hook dir

Enables the use and configuration of hooks in /etc/systemd/system-sleep. Which is used, for instance, by upower to notify the upower daemon of resume.
In order for NixOS to manage the system-sleep directory the systemd Makefile.am must be patched. Otherwise systemd will attempt to create and write to the directory directly during build. Which will fail. Since Makefile.am is patched the systemd build is extended to depend on automake and autoconf.
Well, if this is what upstream upower supplies, we may as well use it. If this ought to be fixed, it should be done upstream.

Fixed the path in bf3fd50 BTW.