"grep -Rs QtXml Modules/"
grep only shows  Modules/ThirdParty/Qt4/otb-module-init.cmake
try deactivating it and see how the build goes tomorrow

This will break ofcourse building external modules for OTB.
But now that we have a test Tu_build_examples, these issues
will not go un-noticed or fixed with a hack!. We find the
bug and fix right way

these variables seem useless in many cases. It was added
during intial superbuild support when CMAKE_PREFIX_PATH
was very less known.

Although there are cases, a force of variable is required,
those are exceptional ones which needs fix on either OTB side
or 3rd party side. Let's do that and avoid these hacks