Fix configure errors when building on 10.9 Mavericks

This fixes the various configure errors that occur when building `php53` or `php54` on OS X 10.9 Mavericks. #591 has a discussion of the changes and their motivations, but the two big changes are:

- `unixodbc` is now required for Mavericks, since OS X's CLT/Xcode doesn't include the required ODBC headers anymore
- `MacOS.sdk_path` is needed almost everywhere because `/usr/include` is empty in Mavericks

These changes fix all the configure errors, but there are still two outstanding issues:

- `php54` doesn't compile without `--without-apache`. With that option, it builds fine.
- `php53` compiles fine, but fails during install without `--without-pear`.

I tested the changes in 10.8 and everything built fine, but I don't have earlier versions of OS X to test them.

I was learning both Homebrew's internals and Ruby on the fly to get PHP to build on Mavericks, so I pre-emptively apologize for any problems: I figured I'd at least PR where I'm at now that it's building.