build clang platform with gcc OpenScientist
Removes the disabling of subdirs when building with clang, since we now have symlinks on cvmfs for the clang11 platform pointing to the gcc10 build of OpenScientist
(Thanks! @bcouturi
additionally, I've fixed a few unused variable warnings in this MR that were now newly flagged in the clang build.
fyi: @rmatev
Edited by Christoph Hasse