This option allows us to avoid venv+pip-install operations. We can use
this in situations where we already know we have all dependencies met
(in a Gentoo ebuild, for example...) and wish to avoid useless work and
potential problems.
I finally took the time to properly learn how to write makefiles. This
was long overdue, but here we go.
Much of the makefile wraps `build.py`, but gradually, we'll extract
stuff from there until the makefile is the main container for build
logic.
