…on the operating system.

This change fixes the build on Mac OS X. According to the original CMakeLists.txt, "rt" is not linked on Mac OS X.

        if (NOT APPLE)
            set (WEBSOCKETPP_PLATFORM_LIBS pthread rt)
        else()
            set (WEBSOCKETPP_PLATFORM_LIBS pthread)
        endif()

The reasons why cppzmq does not depend on the full target of libzmq are:
1. cppzmq is a header-only library, thus it should depend on headers only. Users of cppzmq should provide a libzmq independently, which may be another (locally patched) version.
2. The current libzmq on Bazel registry does not build on Mac M1 (ARM).