[Cin] Two patches from Andrea's testing

[Andrew Randrianasulu]

src:

https://www.cinelerra-gg.org/bugtracker/view.php?id=646

=====
I made two customizations: "--disable-doc" to keep the build from
crashing is adding "AC_CONFIG_MACRO_DIRS([m4])" in configure.ac
otherwise autogen.sh doesn't work. The second one has been years in
the making.
====

I just re-run autogen.sh after first patch, second one is untested yet
(need full rebuild for this, rn I have shared (system) ffmpeg build
for recompilation speeeeed.
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