GitLab

fix: include PYTHON_IS_DEBUG

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

Currently select_cxx_standard(), which sets PYBIND11_CPP_STANDARD when
not externally set, is only called from pybind11_add_module(), but the
embed target setup (which runs unconditionally) makes use of
${PYBIND11_CPP_STANDARD}, which isn't set yet.  This commit removes the
`select_cxx_standard` function completely and just always runs the
standard detection code.

This also tweaks the detection code to not bothering checking for the
`-std=c++11` flag when the `-std=c++14` detection succeeded.

All targets provided by pybind11:

* pybind11::module - the existing target for creating extension modules
* pybind11::embed - new target for embedding the interpreter
* pybind11::pybind11 - common "base" target (headers only)

Makes room for an eventual pybind11::embedded target.