ENH: Pass Qt CMake variable using "mark_as_superbuild"

For sake of consistency, this commit update the system so that both
Qt4 and Qt5 variable are pass to the CTK inner build using the
same mechanism.
Fixes configuration when enabling PythonQt wrapping

This commit fixes the incorrect Qt version check
introduced by commit 47b34216.
Merge branch 'fix-configure-with-pythonqt-wrapping'

* fix-configure-with-pythonqt-wrapping:
  Fixes configuration when enabling PythonQt wrapping
  ENH: Pass Qt CMake variable using "mark_as_superbuild"