On my macOS installation `pkg-config --libs flac` doesn't include Ogg,
and the linker won't search for the library automatically. This means
that FLAC detection fails due to unresolved, Ogg-related symbols.
Fixes#3439.
Newer libFLAC in MXE require a preprocessor definition of
`FLAC__NO_DLL` to signal that libFLAC wasn't built as a DLL. Otherwise
the linker will look for the wrong symbol names.
