Add cmake option for coverage support

Change-Id: I58a2ee2a7d5d6f030a53a251dd2f0f92acf72cc3
Reviewed-on: http://review.couchbase.org/42541
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>