Introduce "make doctor" to diagnose build environment problems

When a user has problems with their build environment that trips up their builds, the cause is often one of just a few "popular" gotchas. For us working with the build system, we've seen them all before, but for the individual user, it's often the first time, and it can be really frustrating to debug.

The Doctor will help users detect if any of these well-known problems is present. Not all issues can be tested automatically, but enough of the most common problems *can* actually be checked, which makes this a worthwhile exercise.

(This was inspired by the macOS Homebrew "brew doctor".)