Add missing dependency to autoconf

[kit-ty-kate]

Sorry I forgot about this one in the last PR

[kit-ty-kate]

Also it seems that the dev version now requires OCaml >= 4.04. Should I add that to the opam file ?

# /home/opam/.opam/system/lib/ffmpeg/libffmpeg_stubs.a(swresample_stubs.o): In function `convert_to_float_array':
# swresample_stubs.c:(.text+0xa94): undefined reference to `caml_alloc_float_array'
# /home/opam/.opam/system/lib/ffmpeg/libffmpeg_stubs.a(swresample_stubs.o): In function `convert_to_planar_float_array':
# swresample_stubs.c:(.text+0xe51): undefined reference to `caml_alloc_float_array'

[toots]

Merging the depext. As for the OCaml versioning, I'll let @gndl comment. He's leading the dev on this now and perhaps he wants to do something else like having a backward compatible implementation.

[gndl]

@smimram, do you need OCaml backward compatibility (I remember you did this commit for this purpose). I agree to define the required OCaml version to 4.04 if it suits everyone or replace caml_alloc_float_array by caml_alloc if it is necessary.

[toots]

I think we can go with >= 4.04 too. Unless @smimram objects, feel free to go ahead this way (he might be busy these days).

[smimram]

I'm fine with dropping backward compatibility a bit. In those days opam did not exist :)

As a side note Debian stable has OCaml 4.02.3, but if it's too much of a hassle to stay compatible with that then go for 4.04.