Add libqdldl recipe

[traversaro]

This packages the qdldl library for C as its own package, to be used as a dependency of libosqp (see conda-forge/libosqp-feedstock#3) and libscs (conda-forge/scs-feedstock#18) differently from osqp and scs, in the qdldl case the corresponding python package is called qdldl-python (https://github.com/conda-forge/qdldl-python-feedstock) so we could have called this recipe directly qdldl, but I prefer to remain coherent with the libosqp and the future libscs packages names. If there is any other preference, I would be happy to switch the recipe name to qdldl.

@h-vetinari I added you as a maintainer given that you are already a mantainer of osqp and scs, let me know if this is ok for you.

I included a patch to permit to only build a shared library and to support building shared libraries on Windows. I did not submit this upstream for now as there are already PR with similar goals but different and slightly more complex (osqp/qdldl#38 and osqp/qdldl#36), if those end being accepted upstream and integrated in the release, I will remove this patch and use the options provided in those PRs.

I did not assumed any kind of ABI compatibility policy on patch releases, as none is documented and the system project osqp had API/ABI breakage in minor releases in the past.

Checklist

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[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipes/libqdldl) and found it was in an excellent condition.

[traversaro]

@conda-forge/help-c-cpp The PR is now ready for review.

[h-vetinari]

Thanks for the ping, I'd be willing to join! :)

[traversaro]

Thanks for the ping, I'd be willing to join! :)

Great! And thanks for the comment, I did not noticed that a Windows CI test was failing, it should be fixed now.

[traversaro]

@conda-forge/help-c-cpp The PR is now ready for review.

Sorry for being a bit premature before, but now the build passes correctly and the recipe is ready for review.

[xhochy]

Not needed, part of CMAKE_ARGS.

Suggested change

	-DCMAKE_PREFIX_PATH=$PREFIX \
	-DCMAKE_INSTALL_PREFIX=$PREFIX \
	-DCMAKE_INSTALL_LIBDIR=lib \

[traversaro]

Great, I was not aware of it. Do you have a pointer on where CMAKE_ARGS is defined so that I can check with arguments are already there?

[traversaro]

I found it: https://github.com/conda-forge/ctng-compiler-activation-feedstock/blob/6290f55b73a887086b0efe0cdb32553646696a8a/recipe/activate-gcc.sh#L144 .

[traversaro]

Done in bc99b66 .