build: VS2015 failure (ICU) back port from io.js

[srl295]

nodejs/node#2279 will affect 0.12+ as well under VS2015 with Intl on.

Basically, VS2015 broke ICU. We'll fix it in ICU 56.

It should apply cleanly, but I couldn't get VS2015 to build with joyent/node/master.

I will open a PR anyway, but will need some help testing.

NB this also adds a general mechanism for floating patches on top of ICU.
@orangemocha

[joaocgreis]

Fixed by 4f24563.