Improve compiling experience on MinGW platform.

[cr1901]

This PR updates the source and Makefile to support MinGW/Msys2 compilation more naturally. In particular:

• Use pkg-config on Windows if the Makefile detects we are using Msys/MinGW.
• Use usb.h if MinGW is detected, as is standard on *nix systems.

Been sitting on this PR for a while now (since June 2017) in a private branch, but only now (after some improvements have been made to the Makefile :)...) I've remembered to prepare it :D!

[dlbeer]

Looks good to me -- thanks for that.