[PROJ] RFC 7: Drop Autotools, maintain CMake

[Mike Taves]

Hi all,

I've prepared a draft RFC that proposes to drop Autotools for PROJ
9.0, and to maintain CMake for build automation, testing and
packaging.

See https://github.com/OSGeo/PROJ/pull/2880

Note that PROJ 9.0 is planned for 1 March 2022.