Until now ostree checked for libsystemd and enabled
support for it if it found it. This commit changes that
behavior by adding an option to enable/disable libsystemd.
This is especially useful if one uses a source based distro
(like Gentoo/Exherbo), where one wants to avoid such automagic
detection of dependencies and prefers switches for that instead.
Closes: #1490
Approved by: cgwalters
