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.

This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

Eliminate dependency on io_thread_get().

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

Since version 2.32, g_thread_init() is deprecated and a no-op.  Let's
upgrade that (optional) dependency so we can remove all those
g_thread_init() calls.

Auto-stop the IO thread in all error handlers.

Move code from the "mad" plugin.

Move code from the "mad" plugin.

Move and refactor code from the Wavpack decoder plugin.

Eliminate duplicate code.

Replaces the method Close().