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.