make: handle when /dev/stderr doesn't exist (direnv#491)

GNUmakefile

@@ -127,7 +127,7 @@ test-go: | $(base)
 	cd "$(base)" && $(GO) test -v ./...
 
 test-go-fmt:
-	[ $$($(GO) fmt | tee /dev/stderr | wc -l) = 0 ]
+	[ $$($(GO) fmt | go run script/both/main.go | wc -l) = 0 ]
 
 test-shellcheck:
 	shellcheck stdlib.sh

script/both/main.go

@@ -0,0 +1,25 @@
+// a little script that outputs stdin back to both stdout and stderr
+package main
+
+import (
+	"os"
+	"io"
+)
+
+func main() {
+	buf := make([]byte, 2048)
+
+
+	for {
+		size, err := os.Stdin.Read(buf)
+		if err != nil {
+			if err != io.EOF {
+				panic(err)
+			}
+			return
+		}
+
+		os.Stdout.Write(buf[0:size])
+		os.Stderr.Write(buf[0:size])
+	}
+}