win32: fix warning and source list display in cmake
add discid_get_support API (string array)

This is part of the implementation for LIB-23
http://tickets.musicbrainz.org/browse/LIB-23
add linux features
add feature list for other platforms
test-only: c usage

Granted, this uses an implementation feature that I had in mind,
but didn't state anywhere:
The location of the features in the array should be fixed.

However, this uses "magic numbers", which is no real solution.
This needs at least another set of defines.

Yes, I realized while implementing that there is no way
I want to be checking if a string is in the list.
That really is ugly in C, compared to other solutions.