https://bugs.gentoo.org/936138
```
libmpdemux/muxer_lavf.c:395:90: error: passing argument 6 of 'avio_alloc_context' from incompatible pointer type [-Wincompatible-pointer-types]
  395 |         priv->oc->pb = avio_alloc_context(priv->buffer, BIO_BUFFER_SIZE, 1, muxer, NULL, mp_write, mp_seek);
      |                                                                                          ^~~~~~~~
      |                                                                                          |
      |                                                                                          int (*)(void *, const uint8_t *, int) {aka int (*)(void *, const unsigned char *, int)}
In file included from /usr/include/libavformat/avformat.h:319:
/usr/include/libavformat/avio.h:420:25: note: expected 'int (*)(void *, uint8_t *, int)' {aka 'int (*)(void *, unsigned char *, int)'} but argument is of type 'int (*)(void *, const uint
8_t *, int)' {aka 'int (*)(void *, const unsigned char *, int)'}
  420 |                   int (*write_packet)(void *opaque, uint8_t *buf, int buf_size),
      |                   ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```
--- a/libmpcodecs/ad_spdif.c
+++ b/libmpcodecs/ad_spdif.c
@@ -54,7 +54,7 @@ static int read_packet(void *p, uint8_t *buf, int buf_size)
     return 0;
 }
 
-static int write_packet(void *p, const uint8_t *buf, int buf_size)
+static int write_packet(void *p, unsigned char *buf, int buf_size)
 {
     int len;
     struct spdifContext *ctx = p;
--- a/libmpdemux/muxer_lavf.c
+++ b/libmpdemux/muxer_lavf.c
@@ -77,7 +77,7 @@ const m_option_t lavfopts_conf[] = {
 	{NULL, NULL, 0, 0, 0, 0, NULL}
 };
 
-static int mp_write(void *opaque, const uint8_t *buf, int size)
+static int mp_write(void *opaque, unsigned char *buf, int size)
 {
 	muxer_t *muxer = opaque;
 	return stream_write_buffer(muxer->stream, buf, size);