Fix build breakage with cmake 2.8

Change-Id: Ida32c782f6e7bf09bb3f16447be9092c15854986
Reviewed-on: http://review.couchbase.org/42542
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>