In libFLAC 0.3.4 (commit c9530118a44), the "dllimport" check has been
changed from "_MSC_VER" to "_WIN32", and now the MPD build is affected
by it.
Defining FLAC__NO_DLL disables the use of "dllimport", which allows
linking properly to the static libFLAC build.
71cd6e6248