Try to extrapolate PYTHON_SO_ABI_FLAG from the SOABI Python config variable

[github at samba.org]

New comment by abartlet on Samba Github repository

https://github.com/samba-team/samba/pull/110#issuecomment-446889829
Comment:
I'm going to close this for now, as we are closing out our old pull requests.  This worm has turned so many times I don't see any good solutions any time soon, but hopefully the 4.10 release will help us see some clarity. 

Please re-raise any practical and agreed solutions as a GitLab merge request.

Thanks!