@gmorph, with great thanks to you, I had success building ardupilot code in a Linux environment, both APMrover2-v2.px4 using make, and ardurover.px4 using waf.
At the end of the day I think the critical step for me was to do Tools/scripts/install-prereqs-ubuntu.sh -y from ~/ardupilot; previously, though I tried to follow the "Building PX4 for Linux with Make" documentation very carefully, my notes indicate that I ran install-prereqs-ubuntu.sh -y from my ~/ardupilot/Tools/scripts directory.
Incidentally, this step took almost an hour with my ~8MB/min internet connection.
Also, for practice or proof, from ~/ardupilot I successfully did ./modules/waf/waf-light rover --upload to install the firmware on Pixhawk (rather than using Mission Planner as I had in the past).
Following your lead, for both building and uploading, I used waf-light in the ardupilot/modules/waf directory instead of waf at the top of the ardupilot directory as implicitly suggested in the github ArduPilot/ardupilot/blob/master/BUILD/md documentation to which you refer, without fathoming the possible differences in effect.
Anyway, kind thanks for your responsiveness and motivation!