Add ITK patch for Mac, solve bug in superproject

Two things in there:
- patch ITK 4.6 : it was using rpath variables to localize libraries on MacOS, which was not compatible with dtkDeploy -> mac nightly bundles couldn't load any plugins
- patches didn't get applied if the source folder was not there (this meant you had to compile twice to get the patches applied the very first time you downloaded medinria): this PR contains a tentative correction for that