Refactor recursive target logic out of default_post into function

Refactor recursive target logic out of default_post into function

The qmake function prepareRecursiveTarget can now be used both by the
existing logic in default_post, as well as future recursive targets that
will be needed as part of the modularization of documentation builds.

Change-Id: Ibc72c3e224cb57c9f1796de3b04fda9de663dbb4
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>