Replace CMAKE_BINARY_DIR with PROJECT variants

This allows other (CMake) projects to include ourselves as a dependency within
CMake itself.

Change-Id: I6b37fed4050f4167455e016bed1f7fd9528bf2f3
Reviewed-on: http://review.couchbase.org/43630
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>