Problem: zsys logging does not work until we've called zsys_init()

E.g. any logging done before creating a first socket will be lost.
The documentation for zsys_set_logstream claims it defaults to stdout
but in practice it defaults to NULL.

Solution: if s_logstream is NULL, set to stdout, *unless* the caller
is using s_logsystem, then don't touch s_logstream.
Problem: --with-libzmq-include-dir and -lib-dir were broken

* --with-libzmq-include-dir went and added /include once more, which
  is obviously broken (it should point straight to the directory that
  holds zmq.h)

* --with-libzmq-lib-dir did nothing except add itself to the help
  output, and confuse users.

Solution: remove both these, which are redundant with --with-libzmq.
Merge pull request #649 from hintjens/master

Removed --with-libzmq-include-dir and -lib-dir (broken)