lavr: deprecate the entire library

Deprecate the entire library. Merged years ago to provide compatibility
with Libav, it remained unmaintained by the FFmpeg project and duplicated
functionality provided by libswresample.

In order to improve consistency and reduce attack surface, as well as to ease
burden on maintainers, it has been deprecated. Users of this library are asked
to migrate to libswresample, which, as well as providing more functionality,
is faster and has higher accuracy.
Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>

configure

@@ -133,7 +133,7 @@ Component options:
   --disable-swscale        disable libswscale build
   --disable-postproc       disable libpostproc build
   --disable-avfilter       disable libavfilter build
-  --enable-avresample      enable libavresample build [no]
+  --enable-avresample      enable libavresample build (deprecated) [no]
   --disable-pthreads       disable pthreads [autodetect]
   --disable-w32threads     disable Win32 threads [autodetect]
   --disable-os2threads     disable OS/2 threads [autodetect]
@@ -6521,7 +6521,7 @@ check_deps $CONFIG_LIST       \
            $ALL_COMPONENTS    \
 
 enabled threads && ! enabled pthreads && ! enabled atomics_native && die "non pthread threading without atomics not supported, try adding --enable-pthreads or --cpu=i486 or higher if you are on x86"
-
+enabled avresample && warn "Building with deprecated library libavresample"
 
 if test $target_os = "haiku"; then
     disable memalign

doc/APIchanges

@@ -15,6 +15,15 @@ libavutil:     2017-10-21
 
 API changes, most recent first:
 
+2017-xx-xx - xxxxxxx - lavr 4.0.0 - avresample.h
+  Deprecate the entire library. Merged years ago to provide compatibility
+  with Libav, it remained unmaintained by the FFmpeg project and duplicated
+  functionality provided by libswresample.
+
+  In order to improve consistency and reduce attack surface, it has been deprecated.
+  Users of this library are asked to migrate to libswresample, which, as well as
+  providing more functionality, is faster and has higher accuracy.
+
 2017-xx-xx - xxxxxxx - lavc 58.9.100 - avcodec.h
   Deprecate av_lockmgr_register(). You need to build FFmpeg with threading
   support enabled to get basic thread-safety (which is the default build