Race condition with received message causes ZMQ_CONNECT_ROUTING_ID to be assigned to wrong socket

Solution: Add an identifier parameter for local attach to zmq::socket_base_t::attach_pipe

Problem: build system does not play nicely as a cmake subproject

Solution: avoid exporting targets and installing files if neither
BUILD_SHARED nor BUILD_STATIC is set

cmake: Install pkgconfig in correct lib directory.

Solution: Use cmake's libdir variable to ensure that the pkgconfig file
ends up in the correct lib/lib64 directory. This matches the autotools
behaviour.

cmake: Streamline integration allowing config of CMAKE_*_OUTPUT_DIRECTORY

This commit ensures that the variable CMAKE_*_OUTPUT_DIRECTORY
are considered when creating the library.

Setting EXECUTABLE_OUTPUT_PATH and LIBRARY_OUTPUT_PATH variables is deprecated

Switched android platform check to official form

See https://markmail.org/message/5ekhfztchs45lz3n#query:+page:1+mid:jfqgzrmwm37uyrt6+state:results

Problem: stack overflow on win64 #2876

Problem: stack overflow on win64 #2876. Basically zmq::signaler_t::wait suffers from stack bloating the same way socket_poller was found to be bloated due to the large size of fd_set
Solution: replace the fd_set in the routine with an instance of optimized_fd_set_t

* More info on conflate queue

Fix for issue #3162

Problem: can't process ZMTP 3.1 cancel/subscribe commands

Solution: add some msg helpers to parse commands, and check for
subscribe or cancel commands and process them accordingly in the xpub
and xsub classes.

Solution: use the --version-script map on those systems as well, as it
is supported

Solution: do it to silence static analysis warnings

Solution: fix path in Makefile.am

Solution: list them

* update pair doc

Fix clang-tidy configuration

Solution: add missing quotation marks

Solution: use correct double quotes

* Problem: connecter classes do not handle ZMQ_RECONNECT_IVL of -1

Solution: Add guards to prevent a reconnect timer from starting if ZMQ_RECONNECT_IVL is -1
(Issue 3158)

Problem: epoll_t employs an unnecessary mutex

Solution: remove the mutex

use cmake compile the unittest

Add cygwin64 CI on Appveyor

wepoll integration fixes

Solution: rename it

Solution: add them so that they get included in "make dist"

Solution: allow failure of cygwin64 build

Solution: do not check for windows.h on cygwin

Solution: add cygwin build

Problem: not able to use wepoll in configure script

Solution: add wepoll as a polling system