All projects now:
* systematically download archives or checkout sources in the top-level directory.
* use variables like EP_SOURCE_DIR, EP_BINARY_DIR or EP_INSTALL_DIR

This commit also:
* updates top-level CMakeLists to explicitly set variables SUPERBUILD_TOPLEVEL_PROJECT
  and EXTERNAL_PROJECT_DIR options instead of relying on their implicit values.
* includes CMake module ListToString where used.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@26528 3bd1e089-480b-0410-8dfb-8563597acbee