Assuming the top-level folder name matches the extension name, it is now
possible to configure Slicer so that it also builds a chosen set of
extension source directories. If a package is generated,
these extensions will also be packaged.

Configuring Slicer passing the option "Slicer_EXTENSION_SOURCE_DIRS" will
achieve the desired goal.

For example, if we want to bundle within Slicer extensions "MinimalistiCli" and "SkullStripper",
the following could be done:

 cmake -DSlicer_EXTENSION_SOURCE_DIRS:STRING=/home/jchris/Projects/MinimalistiCli\;/home/jchris/Projects/SkullStripper .

The associated build tree will then be located in:
  /path/to/Slicer-Superbuild-Release/Slicer-build/Extensions/MinimalistiCli and
/path/to/Slicer-Superbuild-Release/Slicer-build/Extensions/SkullStripper

See http://na-mic.org/Mantis/view.php?id=2293

Fixes #2293

git-svn-id: http://svn.slicer.org/Slicer4/trunk@20576 3bd1e089-480b-0410-8dfb-8563597acbee