icu: build should be silent

[bnoordhuis]

Building the bundled icu-small prints tens if not hundreds of -Wunused-function, -Wsign-compare, -Wstrict-overflow, etc. warnings. These should either be squelched or fixed upstream.

cc @srl295

[srl295]

They should be fixed upstream.

[srl295]

Think it's improved some, remaining filed in http://bugs.icu-project.org/trac/ticket/13001

[bnoordhuis]

Thanks, I'll close out the issue.

[srl295]

^ the ICU ticket has been marked as review. Many but not all of these have been fixed.