Since we switched from autotools to Meson in commit
94592c1406, 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.
The PcmResampler interface is implemented by the two classes
FallbackPcmResampler and LibsampleratePcmResampler. This prepares for
adding more resampler libraries.
