Add CMake option to build WITH_X11

We want to be able to build without X11 support even if some of the used libraries may not work w/o X11 themselves yet or need to be built with X11 support for other reverse dependencies.

HAVE_X11 already exists and is set automagically so far, but using -DCMAKE_DISABLE_FIND_PACKAGE_X11 will break if any dependencies list X11 as required in their cmake config. This is a behavior change as previously it was silently disabled if X11 was not found.

Signed-off-by: Andreas Sturmlechner asturm@gentoo.org