Set the network variable via enviroment variable (closes #3488)

If the environment variable 'DMAKE_NETWORK' is set, use it instead of
guessing the network. Fixes problems with FN network.
Merge pull request #3489 from andrsd/dmake-fn-network-fix

Set the network variable via enviroment variable (closes #3488)
Merge commit '9a8e46cc3746caf94ca601b2fb8d016af5be295c'