COMP: Fix build error in app logic when extension support is disabled.

In vtkSlicerApplicationLogic::IsPluginBuiltIn the variable
Slicer_EXTENSIONS_DIRBASENAME is not defined if the
Slicer_BUILD_EXTENSIONMANAGER_SUPPORT is false.

Also fix a warning due to unused variable slicerRevision
in vtkSlicerApplicationLogic::IsEmbeddedModule in the case
where the Extension Manager was not build.

Co-authored-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

From: Alexis Girault <alexis.girault@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24599 3bd1e089-480b-0410-8dfb-8563597acbee