The certificate of https://gmplib.org/ has expired. The website displays
the following message:
```
YES, THE CERTIFICATE IS EXPIRED. Sorry about that. We're working on the renewal process. Unfortunately, our SSL provider changes the renewal interface each year, and it is always a challenge to figure out what to do. Now the only actions are REGENERATE (actiually not, as the cert is expired), REVOKE (well, not desirable) and INTERMEDIATE CERTIFICATE (which is not an action but a thing, oh well.
```
But this is breaking a lot of builds and this still hasn't been fixed
after a few days, so with this PR I'm instead downloading the libgmp
source archive and bypass the certificate check and manually verify the
sha256 hash of the downloaded file.
Only the `cryptofuzz` and `bignum-fuzzer` projects ideally need to use
the latest repository checkout, instead of a versioned source archive,
because with these projects the aim is to find bugs in libgmp. In all
other projects, libgmp only serves as an oracle, and using the latest
stable version is fine.
Once gmp fixes their certificate, I will revert `cryptofuzz` and
`bignum-fuzzer` to use the repository again. For now it's ok to use the
latest stable version.
These projects were tested and it was found that their use
of MSAN does not work on Ubuntu 20.04. Therefore their use of
MSAN is being disabled.
Related: #6180
* [bls-signatures] Use repository master branch
I merged the BLS functionality into the Cryptofuzz master branch.
Future development will occur there.
* [bls-signatures] Update Boost download location
Guido Vranken