Use mce to prevent suspend while transactions are running

The patch is commit 46e8e26, and commit 197dce4 is just what happens when running `./autogen.sh` with a slightly newer autotools version (it needs running so the `configure.ac` changes are propagated correctly), so commit 46e8e26 is where it's at, and the other one is just necessary line noise.

That this does is that every time transactions become active it starts a new thread that sends CPU keepalive messages to mce via D-Bus. Once all transactions are closed (and PackageKit begins to idle), the thread is stopped and the keepalive stop message is sent just before the thread quits.