* Factor out pybind11/compat/wrap_include_python_h.h

* Fixes to resolve tests_packaging failures.

* Factor out pybind11/compat/pybind11_platform_abi_id.h

* Add pybind11/compat/README.txt and a couple source code comments.

* Minor changes to comments.

* Factor out pybind11/compat/pybind11_conduit_v1.h

* Add long comment to pybind11/compat/pybind11_conduit_v1.h

* Add pybind11/compat/README.txt to wheels.

* Add `-fno-exceptions` to compiler options for exo_planet_c_api

* 1. Move `target_compile_options()` into loop over test targets, in case the `"exo_planet_c_api"` target does not exist.  2. Add `-fno-exceptions` option also for `NVHPC`.  3. Also check for `__cpp_exceptions` in exo_planet_c_api.cpp.

* 1. Fix accident (forgot to undo temporary change).  2. Special-case __EMSCRIPTEN__ in exo_planet_c_api.cpp

* Give up on compiling exo_planet_c_api.cpp with MSVC `/EHs-c-`:

There was one trouble maker (all other jobs worked):

Visual Studio 15 2017:

```
cl : Command line warning D9025: overriding '/EHc' with '/EHc-' [C:\projects\pybind11\tests\exo_planet_c_api.vcxproj]
...
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\include\xlocale(319): error C2220: warning treated as error - no 'object' file generated [C:\projects\pybind11\tests\exo_planet_c_api.vcxproj]
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\include\xlocale(319): warning C4530: C++ exception handler used, but unwind semantics are not enabled. Specify /EHsc
```

* Move pybind11/compat to pybind11/conduit as suggested by @henryiii:

https://github.com/pybind/pybind11/pull/5375#pullrequestreview-2329006001