libzmq-cygwin

Definitive build fixes for cygwin (See https://github.com/zeromq/pyzmq/issues/113 for partial solution)

What’s changed:
./Makefile.am Add cygwin-specific target mostly the same as mingw
./configure.ac Add cygwin-specific target mostly the same as mingw
./tests/testutil.hpp Lengthen socket timeout to 121 seconds

What’s new:
./README.cygwin.md This file
./builds/cygwin Folder for cygwin-specific build files
./builds/cygwin/Makefile.cygwin Makefile for cygwin targets