Tweak Makefile so that xctool can be used.

Caveats: `xcodebuild` seems to exhibit some odd behavior if you specify a -project option, which is required by xctool. `SYMROOT` and related settings get ignored. Manually setting `SYMROOT` when `xcodebuild` is invoked fixes where the built products are placed, but a lot of data is still put into `Derived Data` (and I'd rather not duplicate all those settings).