Previously, these options had two possible values: ON and OFF.
Now, they have three possible values: ON, OFF, and AUTO.
OFF still does what it always did: The feature is just disabled.
With ON and AUTO, we check for the feature. The difference is what
happens when the feature could not be enabled, e.g. because some
dependencies is missing: With AUTO, we just disable the feature (this is
what happened previously with ON). However, with ON, CMake aborts and
prints an error.
Implements: Suggestion by Daniel
https://github.com/awesomeWM/awesome/pull/2665#pullrequestreview-204595138
Signed-off-by: Uli Schlachter <psychon@znc.in>
