BUG: TubeTKMacroAddModules cannot be found in standalone build

Fixing a hack for Prometheus.  Also, Slicer_SOURCE_DIR was not
being propagated down to the TubeTK project, so fill it with the
contents of TubeTK_SLICER_SOURCE_DIR instead.

Change-Id: I231d772870992931fc9f77ca1210362ffd90075a