Python.h check is wrong for Python 3.8

[TinoDidriksen]

https://github.com/divvun/libdivvun/blob/master/configure.ac#L152 checks for ${python_include_path}/Python.h which results in a conftest.cpp that does #include <python3.8/Python.h> - this is not the correct way to check whether Python.h is usable, and never has been. It has worked so far, but Python 3.8 breaks that.

The test must instead add the result of python3-config --includes to the build flags and only #include <Python.h>