The required 'xxhash.h' header can either be in system include directory
or can be copied to 'tools/output'.

The xxh3_128bits based muxer shows no significant slowdown compared to
the null muxer. Decoding times Chimera-AV1-8bit-1920x1080-6736kbps.ivf
with 4 frame and 4 tile threads on a core i7-8550U (disabled turbo boost):

null:  72.5 s
md5:   99.8 s
xxh3:  73.8 s

Decoding Chimera-AV1-10bit-1920x1080-6191kbps.ivf with 6 frame and 4 tile
threads on a m1 mc mini:

null:  27.8 s
md5:  105.9 s
xxh3:  28.3 s