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
STYLE: Use Slicer_SOURCE_DIR instead of TubeTK_SLICER_SOURCE_DIR

Cleaner and eliminates the extra case for Prometheus

Change-Id: Id7442efd0a6a667530c1821b5fef63f9c49b13a4
Merge pull request #261 from chrismullins/fix-module-path-standalone

Fix macro path standalone