setup_zstd.py modified so it is just amending the Extension() kwargs,
but the Extension is initialized by the caller.
this way, amending can happend multiple times (e.g. for multiple
compression algorithms).
also:
- move include/library dirs processing for system-library case
- move system zstd prefix detection to setup_zstd module
- cosmetic: setup.py whitespace fixes
- prefer system zstd option, document zstd min. requirement
