[cmake] Build lldb-server on Darwin, set dep on FreeBSD.
This switches the decision as to whether or not to lldb-server should
be built to check the same flag that was added that controls whether
or not it is added as a dependency to the 'lldb' target.
It also sets that flag on FreeBSD to maintain parity with the existing
build configuration / situation on FreeBSD.
Reviewers: labath, emaste, tfiala
Differential Revision: http://reviews.llvm.org/D12925