[Cin] minor configure.ac changes.

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!