On Windows, clang-cl does not understand /MP.
```
clang-cl: warning: argument unused during compilation: '/MP' [-Wunused-command-line-argument]
```
with Clang 10.0.0
* feat: setup.py redesign and helpers
* refactor: simpler design with two outputs
* refactor: helper file update and Windows support
* fix: review points from @YannickJadoul
* refactor: fixes to naming and more docs
* feat: more customization points
* feat: add entry point pybind11-config
* refactor: Try Extension-focused method
* refactor: rename alt/inplace to global
* fix: allow usage with git modules, better docs
* feat: global as an extra (@YannickJadoul's suggestion)
* feat: single version location
* fix: remove the requirement that setuptools must be imported first
* fix: some review points from @wjacob
* fix: use .in, add procedure to docs
* refactor: avoid monkeypatch copy
* docs: minor typos corrected
* fix: minor points from @YannickJadoul
* fix: typo on Windows C++ mode
* fix: MSVC 15 update 3+ have c++14 flag
See <https://docs.microsoft.com/en-us/cpp/build/reference/std-specify-language-standard-version?view=vs-2019>
* docs: discuss making SDists by hand
* ci: use pep517.build instead of manual setup.py
* refactor: more comments from @YannickJadoul
* docs: updates from @ktbarrett
* fix: change to newly recommended tool instead of pep517.build
This was intended as a proof of concept; build seems to be the correct replacement.
See https://github.com/pypa/pep517/pull/83
* docs: updates from @wjakob
* refactor: dual version locations
* docs: typo spotted by @wjakob
Axel Huebl
Henry Schreiner
Wenzel Jakob