Cmake: fix flags when building python bindings

Merged Sebastian Müller requested to merge python_build_flag_fix into meteo_set_functions

Flags for mhm_lib are forwarded to all targets depending on it. In case of the f2py generated wrapper, this can confuse the C-compiler.

See here: https://github.com/conda-forge/mhm-feedstock/commit/1232ed8e68b6912df0555993901ab7a8ee217924

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