[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The former was deprecated with C++14.

According to the C++11 and C++17 standards, both files are identical.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

This is similar to https://github.com/MusicPlayerDaemon/MPD/commit/b177bffa6a5f1b69639b6cd759dcb16fdd76e5e2, in that it fixes the Windows issue of connecting to the open socket. Also, the listen_socket is set to AcceptNonBlock after the connection anyways.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

Apparently, connecting a socket to a loopback address can block on
Windows, and a non-blocking socket will return WSAEINPROGRESS.  This
broken PoorSocketPair() in commit 2119e4fd, which made the socket
non-blocking right from the start.  This fix postpones the
ioctlsocket(FIONBIO) call until after the connect().

Closes #134

Errors are fatal now.  This makes the class a lot easier to use.

Remove the runtime check for eventfd(), hard-code the feature once
it's been selected at compile time.  The class WakeFD is splitted into
EventFD and EventPipe, using WakeFD as a macro diversion.

Pipes aren't really pollable on Windows.
GLib emulates polling on pipes using separate reader/writer threads.
This adds significant overhead and keeps a strong dependency on GLib.

socketpair() function is absent on Windows as well.
We implement it here in a loose way.