Add an option to skip the AkonadiSearchPIM dependency, to simplify Windows build

Summary:
akonadiconsole only needs KCheckComboBox from libkdepim... which in turn
was bringing a whole lot more dependencies. Let's provide a way to build
this without xapian.

Test Plan: Builds on Linux, with and without the option set.

Reviewers: mlaurent, sengels

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D18613