Setting the variable <extension_name>_EXTERNAL_PROJECT_EXCLUDE_ALL to TRUE
prevents Slicer build system from considering the extension dependencies.

This is particularly useful when (1) an extension support to be built as
both a standalone project or a Slicer extension and (2) external projects
from the extension are either not needed when bundled into Slicer or conflicting
with Slicer ones.

Here is an example of output reported when configuring a custom application
setting the variable ShapePopulationViewer_EXTERNAL_PROJECT_EXCLUDE_ALL to TRUE

[...]
-- SuperBuild - GROUPS extension => LAPACK;MeshLib;RigidAlignment;SurfRemesh
-- SuperBuild - ShapePopulationViewer extension => ignore(ITK);ignore(Qt4);ignore(Qt5);ignore(SlicerExecutionModel);ignore(VTK)
-- SuperBuild - First pass
-- SuperBuild - First pass - done
[...]

git-svn-id: http://svn.slicer.org/Slicer4/trunk@27559 3bd1e089-480b-0410-8dfb-8563597acbee