Revisiting the case for CMake as a primary, cross-major-plats, build config

[jcfr (Jean Christophe Fillion Robin)]

This is also relevant in the context of these topics:

• Is there prior discussion around the build system of CPython itself? - #10 by tamas
• What do you want to see in tomorrow’s CPython build system? - #17 by pf_moore

cc: @erlendaasland @zware @brandtbucher @guido @steve.dower @tjreedy @iritkatriel @hugovk @pf_moore @markshannon