Support BOOST_ASIO_NO_DEPRECATED (Linux, pplx)

[rcasellas]

Hi all,

When adding -DBOOST_ASIO_NO_DEPRECATED to my project (we are migrating from old versions), our build fails. Newer ASIO versions deprecate io_service (use io_context), post is a now a free function, a few API changes (e.g. expires_after), etc.

/usr/include/pplx/threadpool.h:75:18: error: ‘io_service’ in namespace ‘boost::asio’ does not name a type; did you mean ‘has_service’?

/usr/include/pplx/threadpool.h: In member function ‘void crossplat::threadpool::schedule(T)’:
/usr/include/pplx/threadpool.h:72:9: error: there are no arguments to ‘service’ that depend on a template parameter, so a declaration of ‘service’ must be available [-fpermissive]

/usr/include/pplx/threadpool.h:78:38: error: class ‘crossplat::threadpool’ does not have any field named ‘m_service’

This specific error would need changes like the ones below but I wonder if there are other parts (exposed to library clients) that may need change and other implications. Thoughts?
Thanks,
R.

   template<typename T>
    CASABLANCA_DEPRECATED("Use `.service().post(task)` directly.")
    void schedule(T task)
    {
        // service().post(task);
        boost::asio::post(service(), task);
    }

    boost::asio::io_context& service() { return m_service; }

protected:
    threadpool(size_t num_threads) : m_service(static_cast<int>(num_threads)) {}

    boost::asio::io_context m_service;