Add cmake option for building the client to disable auto updater

[mmattel]

References: owncloud/client#9814 (CMake: Implement WITH_AUTO_UPDATER)

Distros might not want to have our auto updater enabled, add option to disable it.

Some more build options available that need documentation, see the referenced PR.

@TheOneRing @dragotin fyi

[mmattel]

Note the referenced PR #242 is a prerequisite before addressing the cmake options topic.