Set *current* CMake source dir as include dir

Allows this project to be used in a third_party subdirectory of another
CMake project.

Change-Id: Ie5232d4ae49a08663fffd78c80c76d89fa9e89a1
Reviewed-on: https://code-review.googlesource.com/11594
Reviewed-by: Paul Wankadia <junyer@google.com>