[patch] switch to xxhash for buildid recomputation

[Frank Ch. Eigler]

Hi -

> [...] We know we need at least 0.8.0, but do we need 0.8.1+? Is
> there something bad in 0.8.0 so we cannot require that version?

I don't know.  A x86-64 build with the old f35 xxhash-devel headers
does pass the tests.  My point about buildbots was that it is one way
we can find out.

> [...]
> Looks good (modulo the 0.8.0 vs 0.8.1 thing discussed above).

OK, switching.  Would you like me to push it to the main branch?

> P.S. Do note that sending patches in git send-email or format-patch
> format makes it slightly easier to apply and test locally. Neither git
> am nor b4 will recognize the diff as the proposed commit. You can apply
> it locally with some tweaks, but it isn't really what you are about to
> commit.

OK, FWIW, 

% curl https://inbox.sourceware.org/.../raw | git am -

is happy processing 100% of that patch, except that it puts more of
the email prose into the commit text.

- FChE