Simplify visibility setting:
Simplify visibility setting:

Rather than string-replacing visibility, use the CMake interface.
Do not set visibility in the ROOT case; ROOT can handle that
just fine itself.
ROOT actually does *not* need visibility=default, now that the
few symbols that are required for using cling (apart from clingtest)
are explicitly requested through attributes.

root-mirror/cling,root-mirror/cling,root-mirror/cling,root-mirror/cling,root-mirror/cling,root-mirror/cling