make dllexport work with compilers other than MSVC

the issue is, flac and metaflac exes rely on flac_internal_???_utf8()
procedures from windows_unicode_filenames.c and there is no easy way
to exclude them from exports without breaking things. So export them
explicitly (they are exported anyway w/o this patch), but add a FIXME
note about the kludge in windows_unicode_filenames.c.
4 files changed