gem should install without configuration when zqm is installed in well known places

[Stefan Kaes]

the way extconf.rb is written, installing the gem fails quite often, even though libzmq and headers are installed in well known places.

$ ruby extconf.rb 
checking for zmq_init() in -lzmq... no
extconf.rb:25: Couldn't find zmq library. try setting --with-zmq-dir=<path> to tell me where it is. (RuntimeError)

$ locate zmq.h
/opt/local/include/zmq.h
/opt/local/include/zmq.hpp
/opt/local/var/macports/software/zmq/2.1.7_0+pgm/opt/local/include/zmq.h
/opt/local/var/macports/software/zmq/2.1.7_0+pgm/opt/local/include/zmq.hpp

Having to specify --with-zmq-dir is a nuisance, especially in combination with using bundler.

I'm going to submit a patch.