COMP: Ensure PreventDirWithSpaces CMake variables is passed to inner project.

This commit ensures that spaces can be used in either source or build
directory when configuring Slicer inner project. This is done by passing
down the variable ${PROJECT_NAME}_SKIP_DIR_WITH_SPACES_CHECK

This commit also documents and simplifies usage of
"PreventDirWithSpaces" CMake module.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23360 3bd1e089-480b-0410-8dfb-8563597acbee