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.