allowing cross-compilation from Linux to Android, also unused platform.hpp as a bonus
don't clean in the middle of a build
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
- [x] cross compilation should work
- [x] it shouldn't clean in the middle anymore, messing up bdists
add sample android cross-compile setup.cfg
don't suppress warnings, it's not worth the trouble
Windows fixes
add missing return in check_sys_un
clean cleans conf
fix space in OS X
Merge pull request #312 from minrk/rebuild

progress towards more sensible build config

- [x] shouldn't suppress warnings where it won't work
- [x] manual overrides for a few things
- [x] allow skipping libzmq version check
- [x] cross compilation should work (#227)
- [x] it shouldn't clean in the middle anymore, messing up bdists

closes #227
closes #300