Adjust asymptote and gnuplot doctests to be directory independent.

Adjust asymptote and gnuplot doctests to be directory independent.

Now you can run the doctests from any directory and they will still
find the files they are scanning to test.  Note that SCons may not be
installed in your PYTHONPATH by default:

> From: Steven Knight
> Date: May 28, 2008; 10:01am
> Subject: Re: easy_installed scons not working
>
> ...
>
> In essence, we've tried to make SCons more like a standalone
> application that happens to use Python, instead of a Python
> application that only exists "within" the Python installation itself.
> This separation also allows you, for example, to experiment with
> upgrading your Python version without having to re-install SCons.
>
> ...
>
> Note that the hacking we've done on our setup.py does allow you to
> install into the site-package/ subdirectory if you wish, using an
> extended --standard-lib option.
>
> ...

So you may need to try something like:
  PYTHONPATH=/usr/lib/scons-2.0.1/ python site_cons/site_tools/asymptote.py
to run the tests.