Do not offer CMake-specific wizards if that plugin is not enabled.

Rationale:
    a) For people not interested in CMake, the wizards unnecessarily
clutter the UI. If unhandled, this problem would get worse the more
build systems we support.
    b) If such a wizard is actually used in the absence of the plugin,
nothing visible will happen after it has finished, which is guaranteed
to confuse users.

Change-Id: I070409d3185b11d2e04e33927a97843e60c657ff
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>