setup.py: clean-up, add option to build libaubio inside python-aubio

The setup script now attempts to build the _aubio extension as follows:

• if src/aubio.h is found, use it to generate python/gen/ files

• if build/src/ is found, use it to link python-aubio against libaubio
• otherwise:
  • add all libaubio source (src/*) to the python module sources
  • look for optional dependencies using pkg-config
  • set flags accordingly

• otherwise, look for aubio headers and libraries using pkg-config

This should help building the python module in a virtualenv (#2),
on windows (#55), and allow installing aubio directly with pip.