COMP: Simplify passing of variable to external projects

Instead of manually creating list of option to give to each project, this
commits removed redundant code and leverage the "mark_as_superbuild"
macro.

By specifying the ALL_PROJECTS value, we ensure that the option will be
passed down to all external projects.

By using "ExternalProject_Include_Dependencies", the CMAKE_GENERATOR
option will be automatically passed to each project using it. This commit
removed the explicit management of this parameter.

Finally, since the passing of the _DIR variable is already set by calling
"mark_as_superbuild" in each external project, this commit removed
the explicit management of these.