Don't specify 'z' as an option to tar. Modern version of tar deduce the
type of compression in use, so specifying 'z' explicitly prevents the
use of tarballs which are not gzipped.
Modified build_mingw function not to construct download URL from a base
URL and a tarball filename. Doing it this way is brittle, since the
filename might be set by a Content-Disposition header instead of by the
URL. Download using curl instead of wget, as the F18 version of curl
supportis Content-Disposition properly but wget does not.