Extend the cmake command-line interface to support
cmake -E env [--unset=NAME]... [NAME=VALUE]... COMMAND [ARG]...
This will be useful to run processes with modified environments
without using a shell or a full "cmake -P" script to wrap it.
Extend the RunCMake.CommandLine test to cover success and failure cases.
Inspired-by: Jonathan Bohren <jbo@jhu.edu>
Brad King