Enable Foundation to build against a new libicu

[djones6]

The Swift build system has existing support for building a new version of ICU, via the --libicu flag, or by specifying the libicu=true and install-libicu targets in a preset.

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 swift#7339).

However, the plumbing to make Foundation build against the newly built libicu is missing: it currently picks up the system version, which causes the build to fail with messages such as:

../build/buildbot_linux/foundation-linux-x86_64/Foundation/CoreFoundation/Locale.subproj/CFCalendar.c.o: In function `__CFCalendarCreateUCalendar':
CoreFoundation/Locale.subproj/CFCalendar.c:(.text+0x15f): undefined reference to `ucal_open_55'
../build/buildbot_linux/foundation-linux-x86_64/Foundation/CoreFoundation/Locale.subproj/CFCalendar.c.o: In function `_CFCalendarIsWeekend':
CoreFoundation/Locale.subproj/CFCalendar.c:(.text+0x1eb): undefined reference to `ucal_isWeekend_55'

This PR adds the necessary flags to build against the new libicu. I replicated the style of the existing solution for the Dispatch libraries.

This has no prereqs, however changes to the build-script (swiftlang/swift#11266) are required before it will have any effect.

[spevans]

You may find this old unmerged PR of mine does the same thing: #722

the swift build script is already passing the info through using pkg-config and it was already working for the LDFLAGS, it just needed the CFLAGS updating

[djones6]

@spevans You're right - and your solution is far simpler. Thanks for the heads up!