Squashed 'external/libzmq/' content from commit 0e7124b

git-subtree-dir: external/libzmq
git-subtree-split: 0e7124b65685dc2bbba194151d1a3a0f5b869250
Merge commit '2d338bde8428810f04082cd5d7bda03828cccd70' as 'external/libzmq'