Re-enable building FLAC decoder with autotools

Because who likes linker errors?

platform/unix/exclude

@@ -1,5 +1,4 @@
 \./libraries/luasocket/libluasocket/wsocket.*
-\./modules/sound/lullaby/FLACDecoder\..*
 \./love\.cpp
 \./modules/sound/lullaby/Mpg123Decoder\.cpp
 \./modules/sound/lullaby/Mpg123Decoder\.h