player: make ffmpeg/libav version check non-fatal

We already enforce mpv rebuilds when ffmpeg/libav SONAME changes.

diff --git a/player/main.c b/player/main.c
index f56191a297..4e51aebd66 100644
--- a/player/main.c
+++ b/player/main.c
@@ -387,13 +387,12 @@ int mp_initialize(struct MPContext *mpctx, char **options)
         // Distro maintainers who patch this out should be aware that mpv
         // intentionally ignores ABI in some places where it's not possible to
         // get by without violating it.
-        print_libav_versions(mpctx->log, MSGL_FATAL);
-        MP_FATAL(mpctx, "\nmpv was compiled against a different version of "
-                 "FFmpeg/Libav than the shared\nlibrary it is linked against. "
-                 "This is most likely a broken build and could\nresult in "
-                 "misbehavior and crashes.\n\nmpv does not support this "
-                 "configuration and will not run - rebuild mpv instead.\n");
-        return -1;
+        print_libav_versions(mpctx->log, MSGL_WARN);
+        MP_WARN(mpctx, "\nmpv was compiled against a different version of "
+                "FFmpeg/Libav than the shared\nlibrary it is linked against. "
+                "This could result in misbehavior and crashes.\n\n"
+                "Upstream does not support this configuration.\n"
+                "Please rebuild mpv in case of any problems.\n");
     }

     if (!mpctx->playlist->first && !opts->player_idle_mode) {