Fixes those programs on Windows.

[[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>

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

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 will be used by filters which have internal buffers which need to
be flushed at the end, e.g. the "soxr" resampler.

Avoid GLib.

Replaces Reset() and eliminates the AudioFormat parameters from the
Convert() method.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.