When specifying CPPFLAGS during `./configure`, `sysconfig.get_config_var('CPPFLAGS')` does capture the originally specified flags.  However, when building a C/C++ extension, any flags specified via CPPFLAGS are not present during compilation of extension source files.

Since preprocessor flags (e.g., '-I' rules) may be used during `./configure` instead of `CFLAGS`, it appears that the `CPPFLAGS` captured should also be injected when customizing the compiler executable to be invoked.

I have PR that will be submitted and linked shortly.

I discovered that `Makefile.in.pre` injects include paths for building the interpreter, itself, which should probably not be passed along via distutils for building C/C++ extensions 😕.

https://github.com/python/cpython/blob/842a2f07f2f08a935ef470bfdaeef40f87490cfc/Makefile.pre.in#L101

It seems like more investigation is required in determining the purpose of `CPPFLAGS` with respect to building the interpreter and if `distutils` should be passing along at`./configure` time of the interpreter.

For now, specifying include paths via `CFLAGS` will still suffice until the scope of `CPPFLAGS` is better defined.

Distutils is now deprecated (see PEP 632) and all tagged issues are being closed. From now until removal, only release blocking issues will be considered for distutils.

If this issue does not relate to distutils, please remove the component and reopen it. If you believe it still requires a fix, most likely the issue should be re-reported at https://github.com/pypa/setuptools