bump revision to 5.8.1

- fixed code for detecting number of CPU cores on MacOS and other BSDs
- hack to display "WinXP" for Windows builds with threads disabled
- two missing CMake files in tarball

not sure why we have two copies of the license in here...

rebased "lossy-enhancements" branch with extra verbosity & debugging

update Cool Edit filter documentation

update ChangeLog

fix length update in library-generated WAV headers on big-endian mach…

…ines

update some dates to 2021 and mention dropped WinXP support in offici…

…al binaries

update more copyright dates to 2020 for release

libcrypto breaks Mac (missing openssl/md5.h), so add option --enable-…

…libcrypto

tag version 5.1.0 release