avconv: drop support for building without libavfilter.

Since the mandatory memcpy in vsrc_buffer has been eliminated, there
shouldn't be any significant reason to build without lavfi anymore.

This will make upcoming support for complex filtergraphs easier to do.

Changelog

@@ -15,6 +15,7 @@ version <next>:
 - XBM encoder
 - RealAudio Lossless decoder
 - ZeroCodec decoder
+- drop support for avconv without libavfilter
 
 
 version 0.8:

configure

@@ -1539,7 +1539,7 @@ avdevice_deps="avcodec avformat"
 avformat_deps="avcodec"
 
 # programs
-avconv_deps="avcodec avformat swscale"
+avconv_deps="avcodec avfilter avformat swscale"
 avplay_deps="avcodec avformat swscale sdl"
 avplay_select="rdft"
 avprobe_deps="avcodec avformat"