cmd/go: allow passing -U flag in cgo CFLAGS #34730
Description
What version of Go are you using (go version)?
$ go version go version go1.13 linux/amd64
Does this issue reproduce with the latest release?
Yes, the version seems to be latest
What operating system and processor architecture are you using (go env)?
go env Output
$ go env GO111MODULE="" GOARCH="amd64" GOBIN="" GOCACHE="/home/nyoxi/.cache/go-build" GOENV="/home/nyoxi/.config/go/env" GOEXE="" GOFLAGS="" GOHOSTARCH="amd64" GOHOSTOS="linux" GONOPROXY="" GONOSUMDB="" GOOS="linux" GOPATH="/home/nyoxi/go" GOPRIVATE="" GOPROXY="https://proxy.golang.org,direct" GOROOT="/usr/lib/go" GOSUMDB="sum.golang.org" GOTMPDIR="" GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64" GCCGO="gccgo" AR="ar" CC="gcc" CXX="g++" CGO_ENABLED="1" GOMOD="" 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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build294100251=/tmp/go-build -gno-record-gcc-switches"
What did you do?
Include line: #cgo CFLAGS: -Ufoo_bar into go source code
What did you expect to see?
Compilation with go build should pass.
What did you see instead?
go build terminates with error: invalid flag in #cgo CFLAGS: -Ufoo_bar