Exclude variable name from *_{INCLUDE,LIBRARY}_DIRS variable

Before this commit, the list of include or library directories was
containing the name of the variable to be expand.

For example, the value for "VTK_LIBRARIES" was

  [VTK_LIBRARIES] Appended dirs: VTK_LIBRARIES;/home/jchris/Projects/Slicer-AHM-Superbuild-Release/VTK-build/bin

whereas the following was expected:

  [VTK_LIBRARIES] Appended dirs: /home/jchris/Projects/Slicer-AHM-Superbuild-Release/VTK-build/bin
Better verbose output for "*_{INCLUDE,LIBRARY}_DIRS" variables update

If possible associated the output with the name of the package.

For example "VTK" will be used instead of "VTK_LIBRARIES"
Do not append non-existent directory to "*_{INCLUDE,LIBRARY}_DIRS" variables

This is particularly useful when building against library that are not
always defining a "<project_name>_LIBRARY_DIRS" variable. This commit
avoids appending a variable name to the list of possible directories
that should be considered.

For example, within ITKv4, the variable ITK_LIBRARY_DIRS is not
always set. See here for more details: https://issues.itk.org/jira/browse/ITK-3026
Merge branch 'avoid-varname-in-dirs-variable'

* avoid-varname-in-dirs-variable:
  Do not append non-existent directory to "*_{INCLUDE,LIBRARY}_DIRS" variables
  Better verbose output for "*_{INCLUDE,LIBRARY}_DIRS" variables update
  Exclude variable name from *_{INCLUDE,LIBRARY}_DIRS variable