Make X11 properly optional
All the facilities were already there, only the find_package call was REQUIRED for some reason. We cannot use an optional find_package call here, since that would fail down the line in KF6WindowSystem's cmake module in case it was built with X11 support.
- Drop HAVE_X11, to be replaced by WITH_X11 option
- Improve X11 PURPOSE
Amends 09b988e6
Signed-off-by: Andreas Sturmlechner asturm@gentoo.org