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 eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Move code from the Vorbis and FLAC decoder plugins.

Don't duplicate the buffer just to null-terminate the string.
According to libFLAC API documentation, the string is already
null-terminated.

To replace g_strdup().

Replaces GLib's g_ascii_strncasecmp().