cmd/cgo: go build failed on _cgo_main.c if there's '#cgo CFLAGS: -W -Werror' in cgo preamble

[hzzb]

What version of Go are you using (go version)?

$ go version
go version go1.12.8 darwin/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/hans/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/hans/golang"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/hans/github.com/hzzb/go/src/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/42/38j1_hsd0ql20_c0g6hcgxww0000gn/T/go-build122131844=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

demo code: https://play.golang.org/p/JybuNyX-bqf

cd to the package directory containing above two files and run go build

What did you expect to see?

go build ran successfully and nothing outputted

What did you see instead?

_cgo_main.c:2:23: error: unused parameter 'fn' [-Werror,-Wunused-parameter]
_cgo_main.c:2:61: error: unused parameter 'a' [-Werror,-Wunused-parameter]
_cgo_main.c:2:68: error: unused parameter 'c' [-Werror,-Wunused-parameter]
_cgo_main.c:2:85: error: unused parameter 'ctxt' [-Werror,-Wunused-parameter]
_cgo_main.c:4:41: error: unused parameter 'ctxt' [-Werror,-Wunused-parameter]
_cgo_main.c:6:26: error: unused parameter 'a' [-Werror,-Wunused-parameter]
_cgo_main.c:6:33: error: unused parameter 'c' [-Werror,-Wunused-parameter]
_cgo_main.c:7:23: error: unused parameter 'a' [-Werror,-Wunused-parameter]
_cgo_main.c:7:30: error: unused parameter 'c' [-Werror,-Wunused-parameter]

[gopherbot]

Change https://golang.org/cl/283232 mentions this issue: cmd/cgo: fix unused parameter warnings in generated _cgo_main.c