fix libsecret detection on OS X
correctly check for Xrandr, fixes build on OS X (though cmstest there is pretty useless)
Merge pull request #727 from parafin/macosx

various cmake fixes for OS X, tested by LebedevRI on Debian SID.
usermanual: proofreading thanks to David Vincent-Jones
usermanual: a few further additions
c&r: ignore clicks outside of crop box

please everyone, test this. c&r is a little delicate wrt. changes.