cbdeps: Update jemalloc build to support Debian

Small change to the jemalloc CMakeLists.txt to identify Debian using
it's major version (i.e. ignore the minor version).

Change-Id: Ic65ed18f061ec1c9347343b8020097c2156a140c
Reviewed-on: http://review.couchbase.org/43798
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Dave Rigby <daver@couchbase.com>