COMP: Ensure Python library directory is considered when fixing up package

When the expression "@SLICER_PYTHON_LIB_DIR@" has been introduced
in commit r21252 [1], it was already resolving to an empty
string because since r15463 [2] the file SlicerBlockCTKAppLauncherSettings is
included in the Applications/SlicerApp/CMakeLists.txt.

Both the fixup script and the launcher settings have been updated to
use the variable "PYTHON_LIBRARY_PATH" introduced in the top-level
CMakeLists.txt by commit r16822 [3]

To prevent such error from happening again, a convenient macro named
"_fixup_paths_append" has been introduced. It basically check if the path
passed as parameter exists and append it to the list of paths. If not,
it throw an error.

[1] http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=21252
[2] http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=15463
[3] http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=16822

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22018 3bd1e089-480b-0410-8dfb-8563597acbee