Refactor setup.py to use dynamic prefix and accept venv.

[allan-kg]

Instead of hardcoding '/usr', use the prefix according to the environment.

It also :

• Creates missing directories if necessary (equivalent of mkdir -p)
• Uses target paths as variables instead of hardcoded strings.
  • Differentiating between path, file, and dir is something that I was thinking about when writing and I chose to use path on all of them. If it is preferable to differentiate, please let me know.