[Ns-developers] build stuff and downloading external deps

[Mathieu Lacage]

hi,

While trying to look at some of the nsc build issues, it occured to me
that we do a lot of things when running waf which require network access
and that this can be problematic in a lot of situations (for example,
such as yesterday when the nsc repo server times out) where network
access is spotty. I noticed also that we have quite a bit of different
code to download different bits and pieces:
  - pybindgen if we don't find the right version for python bindings
  - ns-3-dev-traces if we want to run regression tests
  - nsc if we want to use nsc tcp stacks

To summarize, I would like to split all the network access from our
build stuff proper and:

  1) provide a small download.py file (find a better name if you care)
which will attempt to download missing bits and pieces, depending
whether this is a release tarball or a snapshot from the ns-3-dev main
tree. It could have a couple of command-line options which say which
bits to download and/or update

  2) rework the build scripts to not download anything, try to find
things under the current dir, disable the feature if they can't find it
there, and provide a --with-foo=/location switch for each of the
optional stuff to look for these optional features in a separate
location.

If I hear no loud complaints about the basic idea, I will work on that
starting tomorrow. 

Comments and suggestions welcome,
Mathieu