This replaces the output parameter (which is bad API design). As a
side effect, it fixes the bad [[gnu::pure]] attribute added by commit
a636d2127 which caused optimizing compilers to miscompile calls to
that function. "Pure" functions can be assumed to have no output
arguments, so the compiler can assume the function doesn't modify
them.
Closes https://github.com/MusicPlayerDaemon/MPD/issues/1282
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.
