@neillm - got it working. Had to tweak 2 lines at the bottom of the master bx install shell to point to two of your repos.
1. build_from_github thecodefactory libbitcoin-system witness-payments $PARALLEL ${BITCOIN_SYSTEM_OPTIONS[@]} “$@”
2. build_from_travis thecodefactory libbitcoin-explorer witness-address $PARALLEL ${BITCOIN_EXPLORER_OPTIONS[@]} “$@”
% echo 0279BE667EF9DCBBAC55A06295CE870B07029BFCDB2DCE28D959F2815B16F81798 | ./build4/core/bin/bx ec-to-witness
bc1qw508d6qejxtdg4y5r3zarvary0c5xw7kv8f3t4
Matches the test vector from
https://github.com/bitcoin/bips/blob/master/bip-0173.mediawiki#examples