Add ROOT::<library> as alias to <library> if it doesn't exist
Add ROOT::<library> as alias to <library> if it doesn't exist

One common problem with roottest/rootbench when built outside of ROOT
is that target names from within the build become just a library name
when building externally. That is, using e.g. MathMore for linking
will get converted to just -lMathMore when building externally rather
than the full -lMathMore -lMathCore -lImt -lCore when building as part
of ROOT, which takes dependencies into account since MathMore is a
CMake target. However, roottest cannot always use ROOT::<library>
because when it's built as part of ROOT the target doesn't exist. It
only exists when using ROOT externally by calling find_package(ROOT).
This commit adds the alias library to allow using ROOT::<library> both
when building as part of ROOT and externally. That way, dependencies
are always taken into account when linking.

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