The different section are now commented and should be self explanatory.

To summarize the different steps happening when the file is included:
 1) Call find_package associated with the different external project
 2) Include Block allowing to check platform, and discover required flags.
 3) Set CMAKE_C_FLAGS and CMAKE_CXX_FLAGS
 4) Call "Link_directories"
 5) Update CMake module path and include common Slicer CMake macros and functions.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@16394 3bd1e089-480b-0410-8dfb-8563597acbee