bpo-35296: make install now installs the internal API (GH-10665) (GH-10897)

* bpo-35296: make install now installs the internal API (GH-10665)

make install now also installs the internal API: Include/internal/*.h
header files.

(cherry picked from commit f653fd4d950ac092719b6152e38d77c62b443125)

* Windows installer now also install Include/internal/

The Windows installer (MSI) now also install header files of the
Include/internal/ subdirectory.
4 files changed