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
Merge branch 'master' of https://github.com/darktable-org/darktable
Update French translation of the usermanual after git pull upstream master