allowing cross-compilation from Linux to Android, also unused platform.hpp as a bonus
progress towards more sensible config

- [x] shouldn't suppress warnings where it won't work
- [x] manual overrides for a few things
- [x] allow skipping libzmq version check
- [ ] cross compilation may or may not work
- [x] it shouldn't clean in the middle anymore, messing up bdists

Still some work to do.
don't clean in the middle of a build