https://bugs.gentoo.org/771093
--- a/src/driver_mixer_alsa.c
+++ b/src/driver_mixer_alsa.c
@@ -32,7 +32,7 @@
 
 #define SECTION "MODULE MIXER ALSA"
 
-struct driver_alsamixer {
+extern struct driver_alsamixer {
 	char *card;	 /* name of the soundcard (usually "default" is fine) */
 	GString *channels;	/* channel names to use */
 	struct {