Fixing the upstream Spack revision
I was hoping that we could leave the decision on the exact spack revision to use to the user. But it turns out that it is quite a subtle business of finding one that works perfectly. E.g. since the last release, MPFR has changed a patch and therefore the checksum test fails for MPFR and the build fails. I expect such things to happen on a semi-regular basis, so it seems like one needs to go with Spack master. However, that is a master-branch and is not 100% stable. Maybe a good trade-off would be to fix the upstream revision as a submodule in this repository to a revision that has undergone testing.