* Make mkdir.py work unter python3 versions below 3.5
Because early versions of python3 did not have the optional arg "exist_ok" for "pathlib.Path(...).mkdir(...)" a build under this versions will abort with an error message.
This PR will modify the python script so that it works even under python 3.4 (and below).
* Fix: cannot build after #7060 on Win64
* add double-quotes to `compiler.S.flags`
* fix windows-specific processes (`recipe.hooks.linking.prelink.[12].pattern.windows`)
* rewrite processing of "mkdir" and "cp" in python because of platform-independence
* make consistent with the use of quotation marks in other *.py files
