I am mistaken, but for another reason. The cause of the build
failing was that
./mach build testing/geckodriver is not hooked
up as a dependency to
./mach configure. This causes some
mozilla-config.h problem that again causes bzip2-sys to not compile
successfully. I examined bzip2-sys, and it ships/vendors libbz2-dev,
so the problem here was not that libbz2-dev was missing.