configure: Enable avconv filter dependencies automatically

This makes sure minimal configurations such as
"--disable-everything --enable-avconv" will enable the filters
necessary for running avconv, instead of just keeping avconv
disabled (even if the user specified "--enable-avconv").
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -1742,9 +1742,9 @@ avresample_deps="avutil"
 swscale_deps="avutil"
 
 # programs
-avconv_deps="avcodec avfilter avformat avresample swscale
-             aformat_filter asyncts_filter
-             format_filter fps_filter scale_filter setpts_filter"
+avconv_deps="avcodec avfilter avformat avresample swscale"
+avconv_select="aformat_filter asyncts_filter
+               format_filter fps_filter scale_filter setpts_filter"
 avplay_deps="avcodec avformat swscale sdl"
 avplay_select="rdft"
 avprobe_deps="avcodec avformat"