Blake3 unconditionally enables C++ support, which unconditionally
requires C++20 when built with cmake >= 3.12, even when this is not
required.
Fixing this does not look trivial, and rather than botching the build,
just require C++20, available from gcc 8.x onward.
Signed-off-by: Yann E. MORIN <yann.morin@orange.com>
Cc: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>
