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.