[PROJ] Dropping autotools on master branch

[Mike Taves]

Hi all,

This is a notice to developers that are using PROJ for build scripts.
We are about to finally drop autotools out of the master branch, so
ensure that PROJ is built using CMake 3.9 or newer.

PROJ 9.0 will only support CMake
(https://proj.org/community/rfc/rfc-7.html). Expect to see at least
one release candidate in February, and a final release around 1 March.

Cheers,
Mike