The 'install' command does nothing if the package is detected to be
already installed, but there are circumstances where we will want to
trigger a rebuild and reinstall explicitly. Separate out into two
commands, one of which is a superset of the other.
We need to distinguish between tools we compile to use as part of the
build process, and things we build for the eventual target. Install
these into separate install directories; for now, pkg-config is the
only one of these.
