cmd/go: handle cgo pkg-config pragmas
cmd/go: handle cgo pkg-config pragmas

Fixes issue 2681.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5540047

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go