[Cin] minor configure.ac changes.

[Andrew Randrianasulu]

First one adds no-unknown-warnings cflag back especially if clang was
requested (reduces amount of clang chatter)

second one address (hopefully) pkg-config's loud complains about missing
libs or headers during configure. I tested it briefly on Android/termux and
it seems to work, but better testing much wished for.

If Phyllis will like second patch I can add the same '2>/dev/null'  to
cinelerra/Makefile, where same pkg-config added by me complains
(harmlessly) about not found libs.

Thing is, I prefer detailed output in case something goes wrong. But I do
not mind silencing some parts I added in hope my memory will serve me!
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