go/build: allow colon in #cgo flags
go/build: allow colon in #cgo flags

This makes it possible to say -I c:/foo on Windows.

Fixes issue 2683 comment #3.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5540043
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go