Change 'mark_as_superbuild' API to support multiple project

See #418
Update 'mark_as_superbuild' to support ALL_PROJECTS parameter

See #418
Simplify logic generating argument to pass to ExternalProject

See #418
Ensure CMAKE_OSX_* variable are passed to all projects.

See #418
Merge branch '418-support-use-system-option'

* 418-support-use-system-option:
  Ensure CMAKE_OSX_* variable are passed to all projects.
  Simplify logic generating argument to pass to ExternalProject
  Update 'mark_as_superbuild' to support ALL_PROJECTS parameter
  Change 'mark_as_superbuild' API to support multiple project