Add disable-default-paths flags for hermetic builds

[nh2]

This line sets extra-lib-dirs: /usr/... which doesn't make sense on some systems / Linux distributions, or when you want hermetic builds as on nix.

    if os(linux)
        if flag(openblas)
            extra-lib-dirs:     /usr/lib/openblas/lib
            extra-libraries:    openblas
        else
            extra-libraries:    blas lapack

This PR adds a flag (off by default) to disable the use of these default paths.

[nh2]

CC @peti

[nh2]

As an aside:

At some point in the future, we probably get rid of the hardcoded paths for openblas altogether, because pkgconfig support was just merged into openblas 18 days ago; but it'll take a couple years for this to be reflected in all Linux distributions.

[peti]

IMHO, listing all kinds of system specific paths in the Cabal file is a dubious effort at best, especially if there is no clean way for the user to disable those defaults. It's much better to let the user specify the correct paths for their system with --extra-include-dir and --extra-lib-dir at configure time.

[nh2]

@peti I do understand hmatrix's intentions though: It is convenient for the users to be able to just install it without having to know / learn about the own distribution's paths -- as a nice service kindly provided by the package maintainer.

It will eventually become unnecessary with the upstream project (openblas) providing pkgconfig.

But until then I find it is an acceptably clean solution to have this disable-default-paths flag that disables these defaults.

[nh2]

@albertoruiz A friendly ping :)

[idontgetoutmuch]

This seems very reasonable to me: everyone can carry on without changing anything but users who care (e.g. nix users) can set the flag to true. @albertoruiz do you need any more information from @nh2?

[albertoruiz]

Sorry for the delay!

Thanks to everyone.

[nh2]

Thanks!