Since Intel's default openmp implementation is available in the same src
tarball, we can just include it in the package. This means that `mkl` now "just
works" without any environment variables, fragile setup-hooks, or forced
propagation.
Since the openmp implementation is only needed at runtime (and for test cases),
users can substitute a different one if they prefer by exporting it with
`LD_PRELOAD`, which is how Intel recommends handling this. If they do not do so,
`libiomp.so` lives next to `libmkl_rt.so` and thus will be in the RPATH as a
sane default.
Since this still comes from the same src tarball, we can ship it without losing
the fixed-output derivation; likewise, since Hydra is not building or caching
these, shipping these proprietary packages costs no bandwidth for the nix
community.
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