Enable Foundation to build against a new libicu

[djones6]

This enables Foundation to build against a newly built ICU library, such as when the --libicu parameter to build-script is used.

It may be desirable to be able to do this on Linux in order to match the version of ICU used on the OSX platform, for two reasons:

• Equivalence of behavior across platforms: differences in the implementation of ICU on OSX versus Linux can otherwise lead to different results from Foundation (see: https://bugs.swift.org/browse/SR-5591)
• Performance: the level of ICU that ships on earlier distros such as Ubuntu 14.04 performs poorly when comparing ASCII strings (see stdlib: Improve String comparison performance #7339).

The corresponding Foundation change is in: swiftlang/swift-corelibs-foundation#1144

This PR would enable libicu=true and install-libicu to be added, for example, to the buildbot_linux build preset in the future - however I imagine that those changes, and a method of obtaining a particular version of ICU, probably belong in a separate PR.

I have tested this locally on Ubuntu 14.04 and 16.04, having first downloaded ICU 57.1 and unpacked it at the same level as swift/, swift-corelibs-foundation/, etc.

[djones6]

It turns out, there was an existing PR swiftlang/swift-corelibs-foundation#722 that achieves the same thing in a simpler way, via pkg-config which has already been set up by the build-script.