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- look for windows python lib when using mingw & msys
  - if not found, then look for system python lib as before

pybind11_add_module fails because FindPythonLibsNew.cmake try to link PythonXX.lib, but the first letter "p" should be lowercase.

Since the module is named FindPythonLibsNew, PythonLibsNew_FOUND should be set appropriately.

Found via `codespell`

Make sure `LibsNew` runs correctly if called after the old `Libs`.

* Use LIBDIR and MULTIARCH on linux to find python library

* Remove apple-specific setting; the non-windows one should work fine on OS X

* Default LIBDIR/MULTIARCH to '' (to avoid getting None)

* Remove trailing whitespace from FindPythonLibsNew

Fixes #365. `sysconfig.get_config_var('SO')` already returns the correct
PYTHON_MODULE_EXTENSION, even for debug builds, so there is no need to
add anything else manually.

When run on windows in a venv, PYTHON_LIBRARY pointet to a non-existant
location in the virtual environment directory.

This has been fixed by testing if the path exists and, if not, trying
an alternative path, relative to the PYTHON_INCLUDE_DIR.

If the alternative path doesn't exit as well, an error will be raised.

Also:
- Update documented result variables
- Remove long-ago-deprecated variable