cling travis: add "compiler" tag to GCC-7@Mac build.
cling travis: add "compiler" tag to GCC-7@Mac build.

olifre/root,root-mirror/root,karies/root,root-mirror/root,root-mirror/root,karies/root,karies/root,olifre/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,karies/root,olifre/root,karies/root,root-mirror/root,olifre/root,olifre/root,olifre/root,karies/root,olifre/root,olifre/root,root-mirror/root,karies/root,karies/root,karies/root