Rename macro "ctk_include_once" into "superbuild_include_once"

To update external project files living in your
fork of ExternalProjectsContrib, the ExternalProject migration guide
has been written to help you transition.

See http://www.commontk.org/index.php/Documentation/MigrationGuide/ExternalProject

See #418
Ensure 'superbuild_include_once' works independently of the caller scope.

By using GLOBAL property instead of a local variable, the file will
be properly "include guarded" even if included from within a function.

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

* 418-support-use-system-option:
  Ensure 'superbuild_include_once' works independently of the caller scope.
  Rename macro "ctk_include_once" into "superbuild_include_once"