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
(cherry picked from commit 34b92ffdaa)
setup.py excludes all .c .h .pyx files from installation, but such
docs / license files would still be copied to the target directory
if they are inside the python "borg" package dir.
fixing only the (generic) slice-by-8 crc32 implementation,
it is assumed that CPUs supporting CLMUL can also efficiently
and correctly deal with unaligned accesses.
slice-by-8 is used e.g. on ARM cpus and they might not (efficiently)
support unaligned memory access, leading to bus errors or low
performance.
(cherry picked from commit f9cd6f7512)
