[Cin] configure.ac update

[mnieuw at zap.a2000.nl]

When building on Manjaro, it complained that configure.ac was out of
date, and I should run autoupdate, which I did, and all was fine.
I checked then that updated configure.ac on Mint 19.2 (oldest distro I
have active) and it is fine there too.
I took the opportunity to fix another error: The m4 directory was not
used. This is used when "missing" scripts are installed. 

So I suggest to update the Cinelerra-gg with this.

MatN
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