[release-branch.go1.1] cmd/cgo: pass -Wsystem-headers when looking for errors

««« CL 9120045 / e4f62df3e6c9
cmd/cgo: pass -Wsystem-headers when looking for errors

This works around a bug in GCC 4.8.0.

Fixes #5118.

R=golang-dev, r, minux.ma
CC=golang-dev
https://golang.org/cl/9120045
»»»

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/9259047