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.