The configure.ac.in script uses AC_CHECK_HEADERS for a ton of
things but the source files don't use those conditionals. This
patch adds those. This should fix building on GNU/Hurd.

This patch does runtime checks to see if we can use logind for
suspend/resume same as poweroff/reboot. There's still full support
for ConsoleKit.

based on the xfce4-session implementation [1]
[1] http://git.xfce.org/xfce/xfce4-session/commit/?id=ae28aef315a7a6b90f1649ce6d1f30b842791cbf

Guido Berhoerster assisted by allowing polkit support without
needing systemd.

Eric Koegel changed the xfpm-systemd.c code to use xfpm's internal
polkit code rather than require an external dependancy so the checks
can be done at runtime.