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

Let the constructor and destructor do this.  This means that all users
have to be converted to allocate PcmConvert dynamically.

A few of those tests fail due to bugs.

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.

test/run_storage.cxx depends on EventThread/EventLoop from libevent.a.
Depend on it explicitly. This addresses build failure with
-Dtest=true -Dcurl=disabled -Ddbus=disabled

Closes #396

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!