SCons: clean up the compiler flags a bit.
Fix an unsigned/signed int comparison bug.
Remove support for deprecated AVCodecContext field

Remove support for the deprecated AVCodecContext->quantizer_noise_shaping field,
which as far as I have been able to gather is only relevant for *video* codecs,
and not for audio codecs (the documentation says to "use mpegvideo private
options instead").
README: remove some whitespace