Add CMake option to build WITHOUT_X11

We want to be able to build without X11 support even if some of the
used libraries may not work w/o Qt5X11Extras themselves yet or need
to be built with X11 support for other reverse dependencies.

KONQ_HAVE_X11 already exists and is set automagically so far, but
using -DCMAKE_DISABLE_FIND_PACKAGE_Qt5X11Extras will break if any
dependencies list Qt5X11Extras as required in their cmake config.

Introducing this option means there is no behavior change by default,
cmake will just skip finding Qt5X11Extras or adding unwanted features
if the option is enabled.

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