Add CMake macro add_subdirectory_if_module_enabled

add_subdirectory_if_module_enabled add the subdirectory if the corresponding
ITK module is enabled in the ITK build. This macro is now being used accross
the repository and in the CreateNewExample script.

Change-Id: Iaec6912c5a503e3d6b1f9bad8e8afdc0cd440717
Merge topic 'CMakeMacro' into master

6c584c45 Add CMake macro add_subdirectory_if_module_enabled