added missing dependency program

[rukyr97]

The installation of pkg-config was mandatory to be successful with running the ./ns3 configure command.

[pgawlowicz]

@rukyr97 thanks for this patch. I have applied the two first commits, as I have seen that there are some issues with the last one. Could you fix it? and do another PR?

[rukyr97]

The last commit was to solve an issue I was having when running my ns-3 sim script.
For context, I use symbolic links to develop my work outside of the scratch/ folder in ns-3. This requirement of mine was not compliant with the current ns3-gym version. As far as I could tell model/ns3gym/ns3gym/start_sim.py using os.getcwd(), assumes that my ns-3 script would be placed within this scratch/ folder. The patch was a simple and "dirty" workaround to provide ns3-gym the source directory it should consider, avoiding the use of os.getcwd().

To make it work, simply provide the src_dir param in the Ns3Env constructor.

ns3_gym_env = ns3env.Ns3Env(port=port,
                                simSeed=seed,
                                startSim=True,
                                simArgs=sim_args,
                                debug=True,
                                src_dir="~/foo/bar")

For the content of the variable, I use
src_dir = os.path.abspath(os.path.dirname(__file__)) but again it is specific for my use case and it depends on what the user may need :)

[rukyr97]

Actually, I was just refreshing my memory and now I remember that this PR has some issues, but #72 fixes them, iirc.