Intl: prepare for ICU 56 bump

[srl295]

Even if we don't yet make ICU 56 default as in #2917 I'd like to make it so that people who manually choose 56 won't get a failure.

A file was renamed from derb.c to derb.cpp and so currently a build issue results in this file not being properly excluded. Since it's not explicitly built, it's fine to just list both the .c and .cpp versions in the exclude list.

[srl295]

(by the way, this was caught by the continuous integration of ICU-trunk with Node-master, as discussed at the collaborator's summit.)

[srl295]

@mhdawson fyi

[srl295]

Unfortunately this fix was incomplete. It only works for the full-icu case. For small-icu it missed some other .c to .cpp changes.

Given the time, just fix the rest of the issues in #2917

Looks like a false alarm / unclean local build dir. Clean build worked fine.