Vendor the PortAudio dependency

[dechamps]

FlexASIO capabilities and behaviour tends to be highly dependent on which PortAudio version it's running. For this reason, it would make more sense to "vendor" the PortAudio dependency (i.e. bring it into source control) so that we have more control over the PortAudio library that FlexASIO ships with, and the whole FlexASIO-PortAudio system is bundled as a coherent whole.

This is pretty much a requirement for using PortAudio versions other than the latest stable release, otherwise things are going to be hard to understand and manage. The current PortAudio stable release (pa_stable_v190600_20161030) dates back to 2016 and is not exempt from bugs. Vendoring the library would allow us to use more recent code and pull in any fixes we want.

This would probably be easier if FlexASIO was using CMake (see #36), as PortAudio has explicit support for vendoring in its CMake build system.