Print build type in cmake header

Adds the selected build type (Debug/Release/RelWithDebugInfo etc) to the cmake status:

    -- Using cmake version: 2.8.12.2
    -- Installing to /Users/dave/repos/couchbase/server/source/install
    -- Build type: Debug
    ...

Change-Id: I9523ebda790ba448404a515a7564fcb04ee3bb8f
Reviewed-on: http://review.couchbase.org/42598
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Chris Hillery <ceej@couchbase.com>