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>

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.

These are not in filter_plugins[] and have dedicated constructors
anyway.

For easier state management inside filter plugins.

The old struct config_param remains only for top-level string options.

API simplification.  We can now avoid abusing a "size_t*" as
additional return value.

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

Replaces GLib's GError.