uwsgi: optionally depend on brewed Python

uwsgi didn't depend on brewed Python since system's one is enough
during the build process. If uwsgi is used to serve Python apps, though,
brewed Python should be an available option to link Python's plugin
against.
The added --with-python option will use brewed Python to build the
plugin if --build-from-source is used.
uwsgi: update 2.0.6 bottle.