wscript: activate darwin automatically
interfaces/python/demo_sink.py: fix number of arguments
test_*: /usr/bin/env python
run_all_tests: /usr/bin/env python
demo_*.py: /usr/bin/env python
demo_spectrogram.py: improve ticks
build_osx: use system wide installation, remove parts of the path to catch different versions
README.osx: add some instructions for osx