COMP: Add support for CTK_USE_SYSTEM_* options

This commit update CTK build system to support configuring CTK
passing CTK_USE_SYSTEM_* CMake variables.

To do so it introduces a new macro allowing to recursively traverse the
external projects dependency tree.

The traversal is done in two passes. The first pass traverses the complete
tree to set the value of CTK_USE_SYSTEM_*.

The second pass invokes the "ExternalProject_Add" function and displays
informative messages. An external project could be either:

 - Optional: The project is not required considering the current
             CTK options.

 - Ok: The project has just been included. If the suffix "(SYSTEM)" is
       displayed, it means the libraries (and headers) installed on the
       system will be used.

 - Included: The project has already been included because of prior
             requirement.

Support for the following options has been added:

  -DCTK_USE_SYSTEM_ITK:BOOL=ON
  -DCTK_USE_SYSTEM_DCMTK:BOOL=ON
  -DCTK_USE_SYSTEM_OpenIGTLink:BOOL=ON
  -DCTK_USE_SYSTEM_VTK:BOOL=ON
  -DCTK_USE_SYSTEM_KWStyle:BOOL=ON

See #418