Use of ALIAS target magic requires newish cmake

[cyrush]

We need to work out which cmake versions can support BLT's use of ALIAS style targets.

Older versions get tripped up, see this PR for a few examples:

LLNL/conduit#1233

To be clear -- I am not advocating to support ancient versions (3.9), but we should document what is supported and even throw and error to let folks now what is needed if they try an old version.

Is it CMake 3.21+, or?