Didn't add dll folder since it wasn't copied when updating zstd to
1.3.4.
Didn't add extra files from the lib directory (Makefile, README.md, etc)
that were also left out when updating zstd to 1.3.4.
Add new zstd files to setup_zstd.py (DDict was refactored in
facebook/zstd#1388)
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
Donny Ward