backport of [16524], [16525], [16527] : ffmpeg as plugin + fixes

configure.ac

@@ -2632,12 +2632,20 @@ dnl Trying with pkg-config
      AC_CHECK_HEADERS(ffmpeg/avformat.h)
      AC_CHECK_HEADERS(ffmpeg/avutil.h)
      AC_CHECK_HEADERS(postproc/postprocess.h)
+     if test "${SYS}" = "darwin"; then
         VLC_ADD_BUILTINS([ffmpeg])
+     else
+        VLC_ADD_PLUGINS([ffmpeg])
+     fi
      if test "${enable_sout}" != "no"; then
+        if test "${SYS}" = "darwin"; then
             VLC_ADD_BUILTINS([stream_out_switcher])
+        else
+            VLC_ADD_PLUGINS([stream_out_switcher])
+        fi
      fi
      VLC_ADD_CFLAGS([ffmpeg stream_out_switcher],[${FFMPEG_CFLAGS}])
-     VLC_ADD_LDFLAGS([ffmpeg],[${FFMPEG_LIBS}])
+     VLC_ADD_LDFLAGS([ffmpeg stream_out_switcher],[${FFMPEG_LIBS}])
      dnl newer ffmpeg have a separate libpostproc
      PKG_CHECK_MODULES(POSTPROC, libpostproc,[
        VLC_ADD_LDFLAGS([ffmpeg],[${POSTPROC_LIBS}])