AdsLib: allow UNIX flavors to build more easily
AdsLib: allow UNIX flavors to build more easily

Cherry-picked from 286a06297d5bbd045e354cefa24621baf88d0c6b [286a062]

- simplify #ifdef; only win32 without cygwin is different...
- #pragma once should be available on all platforms
- don't depend on bash and it's features

MrLeeh/pyads