pyconfig.h is generated when running ./configure and it contains the features
that are available on the build machine. This is copied from a local build.
In the original build, there are two kinds of sources: for core functionality
and for extensions. Each of them has a different set of compiler flags. In
order to emulate the same behavior, we define two libraries: 'python3-core' and
'python3-extensions'. Please notice that we still keep a 'python3' library as
the main logical library.