hotfix: protobuf-c compilation failure

protobuf-c does not compile anymore.

Let's handle this a bit better.

We now install protobuf and protobuf-c only for the
flexran agent. That is, if you want to use the flexran
agent, you need to install protobuf/protobuf-c and
you do it this way:

  ./build_oai -I -a

(you add -a)

Other targets don't need protobuf nor protobuf-c, so
it's not installed by the -I command of build_oai,
unless you pass -a with -I.

Also, we now use protobuf 3.3.0, not 2.6.1. The code
has been adapted, a quick test seems to indicate that
the system works, but it has not been intensively tested.