Search for kdelibs optionally

Summary:
QtCurve uses it's own bespoke way of handling optional dependencies
whereby if something is not explicitly enabled it will optionally try to
find a library and update an enabled flag before unconditionally trying
to find the library again.

This currently existed for Qt4&5, but not kdelibs.

Differential Revision: https://phabricator.kde.org/D6395