Don't use absolute path for cmake.exe

Change-Id: Idb0352d6a357175d3809da15a708a2db86013a84
Reviewed-on: http://review.couchbase.org/29583
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>