configure: reject libavcodec >= 55

That future version is expected to remove a number of already
deprecated interfaces, such as CODEC_ID_* or the Snow codec.
VLC still uses these interfaces as of today.

configure.ac

@@ -2223,8 +2223,10 @@ dnl
 AC_ARG_ENABLE(avcodec,
 [  --enable-avcodec        libavcodec codec (default enabled)])
 AS_IF([test "${enable_avcodec}" != "no"], [
-  PKG_CHECK_MODULES(AVCODEC,[libavcodec >= 53.34.0 libavutil >= 51.22.0],
-    [
+  PKG_CHECK_MODULES(AVCODEC,[libavcodec >= 53.34.0 libavutil >= 51.22.0], [
+    PKG_CHECK_EXISTS([libavcodec < 55],, [
+      AC_MSG_ERROR([libavcodec versions 55 and later are not supported yet.])
+    ])
     VLC_SAVE_FLAGS
     CPPFLAGS="${CPPFLAGS} ${AVCODEC_CFLAGS}"
     CFLAGS="${CFLAGS} ${AVCODEC_CFLAGS}"