Update of ROOT version find mechanism
Update ROOT version fine mechanism with the same code that is used in atlas/atlasexternals!1065 (merged) - this update is needed to get the dev3LCG nightlies working again after ROOT changed the location of RVersion.h
(see the MR above for the links in the ROOT code).
I was not able to test this code here for COOL, so I hope the COOL gitlab CI will do this for me.
Tag @mavogel - seems I cannot tag Attila, Reiner and Marcin.