This is useful when trying to bisect an issue in a dependency, and we
want to pull from a specific commit but don't want to have to download
and determine the sha1sum and md5sum of the tarball.
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.
Underneath the hood, CMake uses libcurl and libcurl has had a number of
issues regarding progress feedback. In one sample run against Travis
CI, we ended up with nearly 3,000 lines of progress output for a single
download.
Unfortunately, CMake doesn't have the download and extract steps
separate, so we have some extra work that we have to do. Much of the
content was taken from the ExternalProject.cmake and it's template for
generating the content of the download and extract CMake files.
