packages (net, os/user, runtime/cgo), cmd/link will use internal linking
mode. Otherwise, there are non-standard cgo packages involved, and cmd/link
will use external linking mode. The first rule means that a build of
the godoc binary, which uses net but no other cgo, can run without
needing gcc available. The second rule means that a build of a
cgo-wrapped library like sqlite3 can generate a standalone executable