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.
Also changed -Wno-error=implicit-fallthrough to
-Wno-implicit-fallthrough. For some reason Appveyor's
version of GCC is triggering these warnings despite the
comments, so just shut it up.
