Add systemd .service file for redshift

Add a systemd user .service file to allow users to run redshift as a
daemon using systemd. Also adds a new configure option,
`--with-systemduserunitdir`, which gives the install path.

If set to `no`, systemd support is disabled.
If not set, we use `pkg-config` to find the path.
Add systemd user unit file for redshift-gtk
Merge branch 'systemd-service'

Add systemd user unit files for redshift and redshift-gtk.
Fixes #35, fixes #50.
Update bug report URL to github
Makefile.am: Substitute AM_CPPFLAGS for deprecated INCLUDES
Support for running Redshift without a graphical environment in Linux by using Direct Rendering Manager

Signed-off-by: Mattias Andrée <maandree@operamail.com>
Update Fedora spec file with systemd unit install

Install user unit files into %{_userunitdir}.
Update build requirements.
Fix invalid date in changelog.
Merge pull request #42 from maandree/gamma-drm

Support for running Redshift outside X using libdrm