Export Sanitizer Fix, master branch (2019.11.29.)

I got reminded about this issue (which I've seen in the past already), while working on making the build scripts of atlas/athena a little smarter. So that they could handle the errors seen currently in ATLINFR-3221, a little better. Now the actual story...

While all of the externals projects now provide their own versions of the atlas_export_sanitizer.cmake.in script so that downstream projects would export relocatable cmake configurations tailored to those specific projects, removing the sanitizer from AtlasCMake (Can't even find when I did this. It was a long time ago.) meant that whenever there's any build failure in an externals project, that nightly becomes completely unusable. :-(

This update puts a default sanitizer script back in place, one that the projects of this repository would use.