CMake: Use DTrace when available

This adds support for DTrace probes. This is present on OS X and/or
Linux builds, when available.

Change-Id: I745dc0413fa0bc555112006b6a5449a008696706
Reviewed-on: http://review.couchbase.org/43159
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>