Just use flex-box. And by "use", I mean get ready to study it, change the way you write CSS, give it all of your worldly possessions, and eventually collect money for it at the airport.
?
Certainly, flex-box addresses a problem that's been making CSS writers frustrated for a long time.
Same deal with mobile OS support, really. You have to have a pretty unusual or long-standing audience to support anything pre-4.2, let alone pre-4.0. iOS7 is also probably not worth the time to target for most products.