Fix building without D-Bus (and make it optional)

Summary:
* centralize control in a single var MARBLE_NO_DBUS
  (following pattern of MARBLE_NO_WEBKITWIDGETS)
* move setting option BUILD_WITH_DBUS before finding of deps,
  so it can influence what is searched for
  (not sure if for Android this should be an option at all,
  but keeping for now as is)

Test Plan: Builds with BUILD_WITH_DBUS=ON and OFF for me.

Reviewers: #marble, nienhueser, rahn

Differential Revision: https://phabricator.kde.org/D2147