Improve dbus cross compilation

Building QtDBus on Linux host for QNX target had two issues:
* Configure check failed, because dbus-1 library was not linked in,
  if target platform doesn't support pkg-config.
* Host tools were not built, because pkg-config was not used to locate
  dbus headers on the host.

Task-number: QTBUG-37324
Change-Id: I71d8309599fd40ef2dd8c9e3b44b93a7482019f1
Reviewed-by: Rolf Eike Beer <eb@emlix.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>