fix[cartesian]: MacOS & OpenMP: skip the multithreading

[FlorianDeconinck]

Description

This PR deactivate OpenMP when detecting MacOS (Darwin) as the OS.

The reason are simple apple-clang don't ship 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:

• 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.