COMP: Continuous extension build system also consider Slicer revision updates.

To summarize, the continuous build system now consider two possible updates:
1) Update associated with ExtensionsIndex git repository
2) Change of version associated with Slicer itself

The task (2) is achieved by extracting the revision from vtkSlicerConfigure.h
header file.

moving forward a mechanism preventing the build of extension if Slicer itself
isn't fully build should probably be implemented.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@19982 3bd1e089-480b-0410-8dfb-8563597acbee