Intercept stdout/stderr, and prefix them with the target

Otherwise it's impossible to interpret the source of output.

At the same time, add a differences section to the docs.

Author: rburchell

README.md

@@ -90,12 +90,30 @@ The `GOARCH` placeholder expands to the `GOARCH` under build.
 
 Only a single `output` directive may be found in a package.
 
+# Differences to `go build`
+
+As multibuild is a wrapper around `go build`, most of the behaviour you will see come from there.
+This section is an attempt to document the areas where there are differences, and why.
+
+## Verbose
+
+multibuild adds its own verbose output indicating when different targets start/finish if you pass `-v`.
+
+## Output Prefixing
+
+Output from all builds is prefixed with `GOOS/GOARCH: `, e.g. instead of `go build saying stuff`,
+you will see `linux/arm64: go build saying stuff`.
+
+I think this is generally useful, and the only way to get sane output you can act on,
+so there is no configuration knob to disable it at this time.
+
 ## Cgo
 
 Since the primary purpose of `multibuild` is to cross compile, the use of cgo isn't really
 something I have thought about or focused on: I personally just switch it off and call it a day,
-so that my binaries run in more places. This is one place I have chosen to differ from `go build`:
-`multibuild` will force `CGO_ENABLED=0` by default.
+so that my binaries run in more places.
+
+So `multibuild` forces `CGO_ENABLED=0` by default.
 
 This choice might not be for everyone, though, so `multibuild` will not complain if you explicitly
 choose to enable it, e.g. by running `CGO_ENABLED=1 go tool multibuild`.

cmd/multibuild/multibuild.go

@@ -8,9 +8,11 @@ package main
 //go:multibuild:output=bin/${TARGET}-${GOOS}-${GOARCH}
 
 import (
+	"bufio"
 	"bytes"
 	"encoding/json"
 	"fmt"
+	"io"
 	"log"
 	"os"
 	"os/exec"
@@ -228,9 +230,20 @@ func main() {
 
 func runBuild(args []string, goos, goarch string) {
 	cmd := exec.Command("go", append([]string{"build"}, args...)...)
-	cmd.Stdout = os.Stdout
-	cmd.Stderr = os.Stderr
 	cmd.Env = os.Environ()
+	stdout, _ := cmd.StdoutPipe()
+	stderr, _ := cmd.StderrPipe()
+
+	interceptor := func(source io.ReadCloser, dest io.Writer) {
+		scanner := bufio.NewScanner(source)
+		for scanner.Scan() {
+			line := fmt.Sprintf("%s/%s: %s", goos, goarch, scanner.Text())
+			fmt.Fprintln(dest, line)
+		}
+	}
+
+	go interceptor(stdout, os.Stdout)
+	go interceptor(stderr, os.Stderr)
 
 	if goos != "" {
 		cmd.Env = append(cmd.Env,