Add CMake option to build WITHOUT_X11, drop unused X11 dependency

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.

KRULER_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.

X11 dependency (introduced in 4c7cab8e

)
was not actually used at build time, so the new option will replace it
to determine if the feature is enabled, however libXCB remains
non-required.

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