Just like std::condition_variable, which however has no way to specify
the std::mutex directly.

This replaces the old `dead` flag which was unreliable; it was `false`
if the EventThread was not yet started, which could cause deadlocks in
BlockingCall().

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.

Works around a deadlock bug which happens when waiting for an
EventLoop which isn't running anymore.

Allows eliminating lots of complexity and workarounds for bogus
assertion failures.

Change EventLoop::IsInside() call to EventLoop::IsInsideOrNull().
This means that BlockingCall() may be used during shutdown, after the
main EventLoop::Run() has finished.  This is important because mixers
are currently registered in the main EventLoop.

There is no advantage of using EventLoop::AddCall(), now that
DeferredMonitor is thread-safe.

This change adds two configuration options:

  --with-eventloop=[glib|internal|auto]
  --with-pollmethod=[epoll|auto]

First allows switching between GLib event loop and internal one.
Second chooses backend to use for internal event loop.
Conditional compilation symbols are changed accordingly.
Additional helper macro MPD_OPTIONAL_FUNC_NODEF is added as well.

Implement an event loop without GLib.

Fixes regression from commit 018f4155.

Replaces io_thread_call().  This approach is more generic and easier
to use due to std::function.

No caller needs this.  Fixes use-after-free after returning from
Client::OnTimeout().

Parses CUE data from the "CUESHEET" tag.  Needs further integration in
the update thread.

Pass a callback table to scan_file() and scan_stream(), instead of
returning a tag object.

Based on the APE reader.