suite-sparse: depend on libomp on macOS

[carlocab]

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Upstream relies on OpenMP by default, and even warns about the lack of
thread-safety without OpenMP.

Multiple users have also already asked for this.

Fixes #129011.

[fxcoudert]

Because this depends on OpenBLAS, where OpenMP is provided by GCC, it should do the same.

[carlocab]

That's going to be a problem for dependents, since this has C++ code, and we don't want to mix C++ standard library implementations.

[fxcoudert]

But we don't want to mix OpenMP libraries either…

[carlocab]

Yea, I figured that: #129011 (comment)

Not really sure about the way out other than to make OpenBLAS use libomp too. But that'll require some care as well.

[ywwry66]

make OpenBLAS use libomp too

Because of #50252 (comment), I have been doing this on my own for a while (ywwry66@a47be51). And for my workflow (R user), it has been working pretty well.

Apple is really giving us a hard time :(

[junghans]

And there is #129512, which also fails due to mixing.

[junghans]

@fxcoudert I looked at your comment again(#50252 (comment)). Has anything changed in the last 2 years? What is the status of clang + openMP these days?

[junghans]

I think this is a more general issue and we should track all the packages that are build with gcc only and document why.

I see two main reasons, a.) OpenMP, but that can be mostly be achieved with clang + libomp these days, b.) Fortran and OpenMP, this one is harder, but it would be work a try to build with the new flang (flang-new) compiler.

[fxcoudert]

I don't think things have changed at all since my 2020 comment. Flang is not stable / production-ready. Apple still does not ship libomp with its system compiler.

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