Unified dependencies

Here's the big one.

This work unifies the android/darwin/linux dependencies under one roof.

It builds a completely static binary (except for smbclient on some platforms, which should be fixed up). It also separates gplv3 from the rest (optional on/off), in case that matters at some point in the future.

This is likely to cause substantial breakage over the next few days/weeks, but I think everyone agrees that it's worth the hassle.

Will append full docs this weekend. For now, see the README in tools/depends. Old darwin/rpi/android are left in place for easy reference, we can nuke them soon after merge.

I'd like for this to be the last thing that goes in during this window, which may mean that it slips to the 11th. But Since nightlies may be unavailable for 2-3 days after, I don't want it to block any last-minute merges.