Closes Homebrew/homebrew#22569.

This makes the script much more readable, and also allows us to download
the language packs into the cache where they can be used when installing
aspell.

These methods were overriding CC and causing superenv to potentially
skip the compiler wrapper - especially if no wrapper existed for the
compiler being used, e.g. GNU GCCs.

Closes Homebrew/homebrew#22443.
Fixes Homebrew/homebrew#22249.
Fixes Homebrew/homebrew#22424.
Fixes Homebrew/homebrew#22506.
Fixes Homebrew/homebrew#22521.
Fixes Homebrew/versions#207.

This moves the CompilerSelector fails_with logic into the build
environment setup, making the compiler selection available before
performing actions that depends on knowing what the compiler is, e.g.
setting up PATH.

ENV.setup_build_environment now optionally takes a Formula argument
to provide the information necessary to do the fails_with, and the new
ENV.validate_cc! extracts the fails_with logic from Build.install.

Closes Homebrew/homebrew#22628.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Also adds support for a new `--local` flag, and documentation for the whole command.

Closes Homebrew/homebrew#22378.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Shows a list of built-in commands (but not shortcuts) and searches
for any external commands on the path.

Closes Homebrew/homebrew#22509.

Closes Homebrew/homebrew#22580.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Otherwise specifying arguments means that you can't effectively build just a single commit or pull request as Jenkins will try and override the start commit.