Take advantage of new Makefile in appveyor.yml
- Remove redundant settings by appveyor.yml (now default/automatically set by Makefile)
- Remove 64-bit target.
- Suppress fallthrough warnings. I don't know what's going on here, but despite
-Wimplicit-fallthrough=4
actually getting passed to the compiler, it's still complaining as if it can't see the "FALLTHRU" comments. Disabling that warning is the best I can do.