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

$ go version
go version go1.16.4 windows/amd64

Does this issue reproduce with the latest release?

Yes

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

$ go env
set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\windows\AppData\Local\go-build
set GOENV=C:\Users\windows\AppData\Roaming\go\env
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\windows\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\msys64\mingw64\lib\go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\msys64\mingw64\lib\go\pkg\tool\windows_amd64
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=C:\msys64\home\windows\projects\openssl\go.mod
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\msys64\tmp\go-build390699566=/tmp/go-build -gno-record-gcc-switches

What did you do?

Given test.go and go.mod:

test.go

package test

// #cgo CFLAGS: -I/usr/local/openssl/include -Wno-deprecated-declarations
import "C"

go.mod

module test

go 1.16

Execute this

$ go build -x

What did you expect to see?

WORK=C:\msys64\tmp\go-build908052126
mkdir -p $WORK\b001\
cd C:\msys64\home\windows\projects\test
CGO_LDFLAGS='"-g" "-O2"' "C:\\msys64\\mingw64\\lib\\go\\pkg\\tool\\windows_amd64\\cgo.exe" -objdir "$WORK\\b001\\" -importpath test -- -I "$WORK\\b001\\" -g -O2 "-I/usr/local/openssl/include" -Wno-deprecated-declarations "C:\\msys64\\home\\windows\\projects\\test\\test.go"
...

What did you see instead?

WORK=C:\msys64\tmp\go-build908052126
mkdir -p $WORK\b001\
cd C:\msys64\home\windows\projects\test
CGO_LDFLAGS='"-g" "-O2"' "C:\\msys64\\mingw64\\lib\\go\\pkg\\tool\\windows_amd64\\cgo.exe" -objdir "$WORK\\b001\\" -importpath test -- -I "$WORK\\b001\\" -g -O2 "-IC:\\msys64\\home\\windows\\projects\\test\\usr\\local\\openssl\\include" -Wno-deprecated-declarations "C:\\msys64\\home\\windows\\projects\\test\\test.go"
...