BUG: Fixed mac build error

The problem was that PathExplorer included the Slicer_EXTENSION_CPACK, which conflicted with the superbuild’s Slicer_EXTENSION_CPACK include.

Details of the fixed error (from http://slicer.cdash.org/viewBuildError.php?buildid=208870):

CMake Error at /Users/kitware/Dashboards/Nightly/Slicer4/CMake/SlicerExtensionCPack.cmake:166 (add_subdirectory):
  The binary directory

    /.../SlicerIGT-build/inner-build/SlicerExtensionBundle-binary

  is already used to build a source directory.  It cannot be used to build
  source directory

    /.../SlicerIGT-build/inner-build/SlicerExtensionBundle

  Specify a unique binary directory name.