if err != nil {
			return nil, nil, nil, err
		}
		if goFile != "" {
			outGo = append(outGo, goFile)
		}
		if cxxFile != "" {
			outCXX = append(outCXX, cxxFile)
		}
	}
	return outGo, outC, outCXX, nil
}

// Make sure SWIG is new enough.
var (
	swigCheckOnce sync.Once
	swigCheck     error
)

func (b *Builder) swigDoVersionCheck() error {