Protect against parallel builds better

[carolynvs]

Homebrew now uses a parallel build by default which causes get-deps to run at the same time as the go build (obviously this doesn't go so well). I've temporarily turned off parallel builds in the homebrew build but it would be nice to figure out how to improve the makefile instead.

Essentially nothing can run in parallel with get-deps, but all the other labels, like the cross-builds would benefit from that.