- `brew update` should try to install `curl` before `git` on older
versions of Mac OS X where it is needed for accessing modern SSL
certificates.
- We don't need an HTTP mirror for `git` because `curl` will already be
installed before it is downloaded.
- Don't recommend GCC on Mac OS X versions where it can't be built with
the default system compiler.
- Start using the Homebrew `curl` on Mac OS X versions where it is
needed as soon as it is installed.