I found the dual use of CMAKE_*_PATH variables to make it difficult to
reason about this code. Now a separate set of variables are used to
communicate with the cc wrapper, and less work is performed in the
wrapper itself.

We no longer pass the SDK include path as a -isystem directory on
Xcode-only setups. Doing so is redundant with `--sysroot` and has other
side effects, namely changing the include path search order, which can
break compilation of some software (e.g. qemu).

On Xcode-only 10.9, we can additionally omit `--sysroot`, as the correct
paths are built into the tools.

A new variable, HOMEBREW_SYSROOT, is used to this information to the
wrapper. It will be unset on Xcode-only 10.9. HOMEBREW_SDKROOT will
continue to be set, as it is used for other things besides setting the
include search path.

Closes Homebrew/homebrew#29379.

Closes Homebrew/homebrew#29380.

Closes Homebrew/homebrew#29318.

Closes Homebrew/homebrew#29336.

Closes Homebrew/homebrew#29355.

Replaced with `brew list --versions --multiple`.

The checksum on the tarball is not stable.

Closes Homebrew/homebrew#26671.

Closes Homebrew/homebrew#29315.

Storm requires a specific version of jzma, which could be supplied
as a resource. Pull requests will be reviewed.

Closes Homebrew/homebrew#23283.

Closes Homebrew/homebrew#29294.

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

Closes Homebrew/homebrew#29231.

Closes Homebrew/homebrew#29139.

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

Closes Homebrew/homebrew#17352.

Closes Homebrew/homebrew#27258.

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

Git repositories should always be updated when the ref is a branch.
They should be also updated if no ref was specified or if the ref isn’t
present. This stops the repo being updated if we’re building a specific
revision and it is present. Additionally, this stops the update barfing
on repositories where we’re using a revision and there is no upstream
`master` branch.

Closes Homebrew/homebrew#29218.