func (prog *Program) Build() {
	var wg sync.WaitGroup
	for _, p := range prog.packages {
		if prog.mode&BuildSerially != 0 {
			p.Build()
		} else {
			wg.Add(1)
			go func(p *Package) {
				p.Build()
				wg.Done()
			}(p)
		}
	}
	wg.Wait()
}
func (prog *Program) Build() {
	var wg sync.WaitGroup
	for _, p := range prog.packages {
		if prog.mode&BuildSerially != 0 {
			p.Build()
		} else {
			wg.Add(1)
			go func(p *Package) {
				p.Build()
				wg.Done()
			}(p)
		}
	}
	wg.Wait()
}
// Build calls Package.Build for each package in prog.
// Building occurs in parallel unless the BuildSerially mode flag was set.
//
// Build is intended for whole-program analysis; a typical compiler
// need only build a single package.
//
// Build is idempotent and thread-safe.
//