Revert "build: don't download and extract tarball if source is already present"

This reverts commit 90658982644b76a9e8e4abfbcd16450df26b0a32.

It failed to work correctly on a fresh clone.  See #1433.
build: don't download and extract tarball if source is already present

When building under Homebrew, we want to let Homebrew manage
downloading and extracting the tarballs. See PR 1411.

Also make sure to skip only if directory is not empty. Fix #1433.