Makefile: Use ?= for initial variable definitions
Makefile: Use ?= for initial variable definitions

:= is not capable of accepting values(for variables) from execution
environment. Therefore replace := with ?= for completeness.

Signed-off-by: Anoop C S <52add29bc15c0f3832a373982b132796d33ff6c3@cryptolab.net>

ceph/go-ceph,ceph/go-ceph,ceph/go-ceph