// gccErrors runs gcc over the C program stdin and returns
// the errors that gcc prints. That is, this function expects
// gcc to fail.
func (p *Package) gccErrors(stdin []byte, extraArgs ...string) string {
	// TODO(rsc): require failure
	args := p.gccCmd()

	// Optimization options can confuse the error messages; remove them.
	nargs := make([]string, 0, len(args)+len(extraArgs))
	for _, arg := range args {