Add various fixes to enable us to build a selection of useful USB plugins.
Also, skip tests that don't make sense on WIN32 or that will not work.
With much help from Mario Limonciello <mario.limonciello@dell.com> -- Thanks!
We don't actually need either of the things it provides (looking up in source
and built, and converting to an absolute path) so just replace it with
g_build_filename() instead.
This also has the advantage that it does the right thing on Windows.