`upcxx --help` should only list installed networks

[Dan Bonachea]

Current upcxx --help output includes:

 UPC++ library configuration:
  -network={ibv|aries|ofi|ucx|smp|udp|mpi}
                   Use the indicated GASNet network backend for communication.
                   The default and availability of backends is system-dependent.

It would be more useful if the message from an installed upcxx listed the actual available networks and default for that install.

[Paul Hargrove]

Set upcxx -help network list at install time

This commit resolves issue #598 by customizing the list of networks in the output of upcxx -help at install time to include only those actually supported in the build, and by identifying the default.

→ <<cset 2a330aba122d>>