cmake: fix fragile code related to HAVE_FORK macro

* report from Richard Newton indicated previous solutions was not
  working on Windows
Merge pull request #936 from soundart/master

cmake: fix test_fork,  fix fragile code related to HAVE_FORK macro
Revert "Fix issue with using #if HAVE_FORK instead of #ifdef HAVE_FORK"

This reverts commit 6ad8b59b0ba1a02a0228b2ca077e4fe00f0bf742.
Merge branch 'master' of https://github.com/zeromq/libzmq