meek and moat are broken on macOS 10.9 with the switch to Go 1.12

When using meek and/or moat on macOS 10.9 one gets

dyld: Symbol not found: _unlinkat
  Referenced from: .../Tor Browser.app/Contents/MacOS/./Tor/PluggableTransports/meek-client-torbrowser
  Expected in: flat namespace

and the pluggable transport being non-functional. As I wrote in comment:36:ticket:30126:

I suspect the underlying problem is a change in Go 1.12 where Go is starting to use libSystem.dylib etc. for syscalls: https://github.com/golang/go/issues/17490. (The mac-ports folks had to work around that e.g. in https://trac.macports.org/ticket/58138).

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