Cherry-pick "Merge branch 'PlatformHandlingFix-1.0-20180621' into '1.0'" into master
ATLAS_PLATFORM Handling Update, 1.0 branch (2018.06.21.)
See merge request !317 (merged)
(cherry picked from commit b5945765)
- 8ed2651b Re-worked how ATLAS_PLATFORM would be set up by atlas_project(...).