- unset IFS to fix version parsing on older Bashes
- reuse existing HOMEBREW_MACOS_VERSION if it’s available
- set rather than unset popup_stub

On older versions of OS X (i.e. pre-Mavericks) `xcode-select` is not
required to be set when using the CLT. Fix this behaviour so `brew
update` doesn't tell everyone on that configuration to `brew install
git`.

Mountain Lion: http://opensource.apple.com/source/libarchive/libarchive-25.1/config.h
  `/* #undef HAVE_LIBLZMA */`

Mavericks: http://opensource.apple.com/source/libarchive/libarchive-29/config.h
  `#define HAVE_LIBLZMA 1`

Avoid using an unnecessary `xz` dependency when it's not needed.

Write the README but also a `.travis.yml` file (and in future perhaps a
`Jenkinsfile`).

It looks like a rebase/merge conflict remmenet.

Multiple issues have been filed about this environment variable, hence
give a warning. #932 might make this unnecessary when merged, but
hopefully this warning can be merged quickly.

ruby-macho now performs all relocations in Homebrew.

Additionally, delete the defunct CctoolsRequirement.

It's not used on enough configurations now that there's little point in
keeping it around. See e.g. `:autoconf` for prior art.

This was omitted from #1258, incorrectly fixed in #1267, reverted in
 #1268 and re-applied in this commit.

If you’re using a /usr/local prefix but e.g. /usr/local/homebrew/Cellar
then you’ll miss out on most binary packages for no good reason so warn
people of that.

If HOMEBREW_CELLAR (or HOMEBREW_PREFIX) are children of
HOMEBREW_REPOSITORY it’s important to deny write to the repository and
enable write to the Cellar/prefix afterwards.

The homebrew/homebrew wiki no longer exists.

Fixes #1223.

This is needed since some formulae now set

  ENV["GLIDE_HOME"] = HOMEBREW_CACHE/"glide_home/#{name}"