Introduce an optional dependency on ICU.

Users will be able to build against ICU in order to have full Unicode
properties support.

Change-Id: I22e722392a63fd292f23be221a1e7b807d70c56b
Reviewed-on: https://code-review.googlesource.com/4153
Reviewed-by: Paul Wankadia <junyer@google.com>
2 files changed