build: better error messages

x/clang/clang.go

@@ -17,6 +17,7 @@
 package clang
 
 import (
+	"io"
 	"os"
 	"os/exec"
 )
@@ -26,6 +27,9 @@ import (
 // Cmd represents a nm command.
 type Cmd struct {
 	app string
+
+	Stdout io.Writer
+	Stderr io.Writer
 }
 
 // New creates a new nm command.
@@ -33,13 +37,13 @@ func New(app string) *Cmd {
 	if app == "" {
 		app = "clang"
 	}
-	return &Cmd{app}
+	return &Cmd{app, os.Stdout, os.Stderr}
 }
 
 func (p *Cmd) Exec(args ...string) error {
 	cmd := exec.Command(p.app, args...)
-	cmd.Stdout = os.Stdout
-	cmd.Stderr = os.Stderr
+	cmd.Stdout = p.Stdout
+	cmd.Stderr = p.Stderr
 	return cmd.Run()
 }