openblas: add pkg-config aliases

[timokau]

Motivation for this change

Openblas provides implementations for various standards. To be able to assume the place of cblas for example in the eyes of a program using pkg-config, it needs to provide a .pc file telling pkg-config that it is cblas. Concretely, sage needs these pc files. I included this in sage but now I think this is not really sage specific.

The current implementation adds those pc-files as independent packages that always assume openblasCompat. That is not ideal. Other implementations I could think of:

• add openblasCompat-cblas-pc and openblas-cblas-pc speperately (ugly)
• add the pc as a dependency of openblas. This would always include them and be the easiest option. Not sure if that could break something.
• add the pc as a wrapper around openblas that can optionally be enabled (no rebuild of openblas necessary)

What do you think @ttuegel?

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.

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[matthewbauer]

Would it be possible to just install these 3 files in the openblas derivation? IIUC its harmless to have multiple of them, so might as well aim for maximum compatiblity.

[timokau]

I think that is what I meant by option 2. That would really be the easiest and nicest option. I just wasn't sure if it could be harmful. What would pkgconfig do if it finds multiple options?

[timokau]

If it isn't harmful we may even suggest the inclusion upstream.

[timokau]

Apparently upstream actually used to have this: OpenMathLib/OpenBLAS#1096

[timokau]

Upstream report: OpenMathLib/OpenBLAS#1740

[ttuegel]

I don't see a problem with including these in our OpenBLAS build.

[timokau]

Upstream seems to more or less agree:

I am not opposed to reinstating the additional pkgconfig files, as long as they do not cause any new and interesting problems. Getting the recent thread memory allocating mess fixed is much higher on my list of priorities however.

So I'll amend the PR to add the pkg-config files unconditionally and be based on stable.

[timokau]

Done, but still testing.

[timokau]

Okay, tests (built sage and tested it and its openblas-using dependencies) succeed. Upstream doesn't include it since it would conflict with other blas implementations providing the same pkgconfig files. Since nix's pkgconfig files are not system-wide, I think there is no issue here. A problem would only occur if a package depends on two blas implementations and depends on the blas.pc file. In that case, it would be a simple matter of which blas implementation appears first in PKG_CONFIG_PATH (so probably which one appears first in buildInputs.

So in summary I think this can't hurt. I'll merge this tomorrow if nobody complains.