fix[cartesian]: Disable OpenMP multithreading for DaCe backends

[FlorianDeconinck]

Description

This PR deactivate OpenMP when detecting MacOS (Darwin) as the OS. based on a new configuration.

This was motivated by apple-clang not shipping with openmp by default and the way to acquire it is requires brew or other workaround we won't expect out of our users.

In detail the PR:

• add an GT4PY_CARTESIAN_ENABLE_OPENMP env var and configuration flag
• strips includes and omp_get_max_threads from codegen on the glue code of stencil DaCe backends when all the computes are sequential (non MacOS related),
• put an option in the StencilBuilder to centralize deactivation of OpenMP and turn it off when detecting Darwin (+ warning),
• makes all maps sequential as part of the OIR -> TreeIR process based on the above option,
• strips openmp flags from the command line based on the above option.

Requirements

• All fixes and/or new features come with corresponding tests.

[havogt]

Adding @tehrengruber as reviewer to check with PMAP people if they are running with openmp on mac for development.

[romanc]

I'd also suggest to add minimal tests (e.g. we run daily CI on MacOS hardware) to catch obvious errors like deactivating OpenMP on the wrong platform.

Also, we might wanna allow OpenMP on MacOS systems if used in combination with GCC (like we do - without issues - on the daily CI).

[FlorianDeconinck]

My bad - this was supposed to be a draft. Thanks for the review, I'll be back on it today

[FlorianDeconinck]

PR has been modified to introduce a wider GT4Py flag to deactivate OpenMP

[FlorianDeconinck]

Adding @tehrengruber as reviewer to check with PMAP people if they are running with openmp on mac for development.

This is now not disabled on MacOS by default. Should be transparent to PMAP people

[twicki]

to me this looks like it is ready to go

[romanc]

I find it a little weird that the oir to treeir translator is deciding based on gt_config.GT4PY_CARTESIAN_ENABLE_OPENMP while the pyext builder is looking at gt_config.build_settings["openmp"]["use_openmp"]. I guess it's a matter of taste.

[FlorianDeconinck]

@romanc / @twicki : as discussed this morning - ready for review/merge

[romanc]

LGTM