Using environment variables to set compression/decompression parameters has security implications. Therefore,
we intentionally restrict its usage. Currently, only `ZSTD_CLEVEL` is supported for setting compression level.
If the value of `ZSTD_CLEVEL` is not a valid integer, it will be ignored with a warning message.
+Note that command line options will override corresponding environment variable settings.
#### Long distance matching mode
The long distance matching mode, enabled with `--long`, is designed to improve