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

cases remaining to test:

- [x] Windows + bundled libzmq
- [x] Windows + installed libzmq
- [x] Windows bdist_egg
- [x] BSD + installed libzmq
- [x] BSD + bundled libzmq
- [x] OSX + installed libzmq
- [x] OSX + bundled libzmq
- [x] skip_check_zmq works
- [x] None of this matters to PyPy
- [x] detect sys/un.h works on OSX
- [x] detect sys/un.h works on Linux
- [x] detect sys/un.h works on BSD
- [x] detect sys/un.h doesn't run on Windows
- [x] detect sys/un.h works (won't be found) cross-compiling to android