Code style improvements

Some code style improvements: removed redundant parentheses, added const, joined declaration and assignment of local variables

Use static_cast instead of C-style casts, make local variables const where possible, use switch instead of chained if/else, extract rm_helper_multiple_subnodes from rm_helper to reduce nesting

Problem: wepoll outdated

Simplified and optimized several code fragments

Solution: update to wepoll 1.5.2

* Problem: No CI for mingw64

Adding CI support for mingw64 to appveyor.yml

Reason: To help address the issue:
  Several checks fail under 64-bit MinGW on Windows when running make check #3185

* Problem: Docs do not build with mingw64, dll already in right place

Turning off WITH_DOC for mingw64
Removing copy command for mingw64 and cygwin64 (it now seems to be directly built in "bin" directoy)

Problems: no docs for zmq_timers_*, zmq_timers_* have been DRAFT for a long time

Solution: move them from DRAFT state to STABLE

Solution: add it to the makefile

Solution: add a manpage
Fixes #3005

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