Fix problem with missing ROOTSYS variable

If using a non FairSoft installation of the dependencies problems were reported after the last changes to the build system. Instead of hardcoding the SIMPATH use the the information from ROOT about its binary directory. Remove some old usage of the root-config script, use CMake variables provided by ROOT instead.

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requested review from @e.clerkin

assigned to @f.uhlig

Dear @f.uhlig, @v.friese, @p.-a.loizeau,

you have been identified as code owner of at least one file which was changed with this merge request.

Please check the changes and approve them or request changes.

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approved this merge request

This change fixes my issue. Many thanks.

added 2 commits

• 890f0b2a - 1 commit from branch computing:master
• b2cedb4c - Fix problem with missing ROOTSYS variable

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merged