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* fix: ipo should be off for debug or relwithdebinfo

Closes #2587

* docs: slightly more detailed about IPO

* Update pybind11Common.cmake

* docs: minor cleanup

* ci: fix add-path command

* docs: add example of use in-place

* Update .github/workflows/ci.yml

* feat: import check as a common function

* docs: add cmake to docs

* docs: installing section

* docs: feedback from @wjakob

* 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

* Add vcpkg installation instructions

* Casing

* Move instructions
Co-authored-by: Henry Fredrick Schreiner <henry.fredrick.schreiner@cern.ch>

* feat: FindPython support

* refactor: rename to PYBIND11_FINDPYTHON

* docs: Caps fixes

* feat: NOPYTHON mode

* test: check simple call

* docs: add changelog/upgrade guide

* feat: Support Python3 and Python2

* refactor: Use targets in tests

* fix: support CMake 3.4+

* feat: classic search also finds virtual environments

* docs: some updates from @wjakob's review

* fix: wrong name for QUIET mode variable, reported by @skoslowski

* refactor: cleaner output messaging

* fix: support debug Python's in FindPython mode too

* fixup! refactor: cleaner output messaging

* fix: missing pybind11_FOUND and pybind11_INCLUDE_DIR restored to subdir mode

* fix: nicer reporting of Python / PyPy

* fix: out-of-order variable fix

* docs: minor last-minute cleanup

fix: include PYTHON_IS_DEBUG

* Add AutoWIG to list of binding generators

This variable is a CMake community standard to set the C++
standard of a build. Document it in favor of the previous variable,
which stays as a legacy flag for existing projects.

https://cmake.org/cmake/help/v3.17/variable/CMAKE_CXX_STANDARD.html

* Added note regarding make install

pybind11 headers passed via the `pybind11_add_module` CMake
function can now be included as `SYSTEM` includes (`-isystem`).

This allows to set stricter (or experimental) warnings in
calling projects that might throw otherwise in headers
a user of pybind11 can not influence.

* Doxygen needs `RECURSIVE = YES` in order to parse the `detail` subdir.

* The `-W` warnings-as-errors option for sphinx doesn't work with the
  makefile build. Switched to calling sphinx directly.

* Fix "citation [cppimport] is not referenced" warning.

This updates the compilation to always apply hidden visibility to
resolve the issues with default visibility causing problems under debug
compilations.  Moreover using the cmake property makes it easier for a
caller to override if absolutely needed for some reason.

For `pybind11_add_module` we use cmake to set the property; for the
targets, we append to compilation option to non-MSVC compilers.

[skip ci]

Under MSVC we were ignoring PYBIND11_CPP_STANDARD and simply not
passing any standard (which makes MSVC default to its C++14 mode).

MSVC 2015u3 added the `/std:c++14` and `/std:c++latest` flags; the
latter, under MSVC 2017, enables some C++17 features (such as
`std::optional` and `std::variant`), so it is something we need to
start supporting under MSVC.

This makes the PYBIND11_CPP_STANDARD cmake variable work under MSVC,
defaulting it to /std:c++14 (matching the default -std=c++14 for
non-MSVC).

It also adds a new appveyor test running under MSVC 2017 with
/std:c++latest, which runs (and passes) the
`std::optional`/`std::variant` tests.

Also updated the documentation to clarify the c++ flags and add show
MSVC flag examples.

Makes room for an eventual pybind11::embedded target.

See the documentation for a description of the options.