This uses the nixpkgs SDK to build, which means we can get rid of a lot
of our build hacks. This does mean we're now assuming that Xcode is in
`/Applications/Xcode.app` instead of letting xcode-select find it,
though this is configurable.
Reenable python3 support since using the nixpkgs SDK fixes the
incompatibility there.
Switch lua/perl/python/ruby/tcl support away from dynamic since vim
isn't finding the right libraries anymore, for some reason.
Remove some of the other old stuff like bits of the patchfile that don't
seem to be needed anymore.
