cmake: find DirectFB through a cmake module + use it in installed sdl2 prefix

[madebr]

project:

• let test/versioning.sh also check the version variables embedded in the generated configure script
• Switched order of regenerating configure and running test/versioning.sh in docs/release_checklist.md

directfb:

• Find directfb headers using a FindDirectFB.cmake module.
• Re-use this module for looking for directfb for an installed sdl2 prefix

Addresses cmake directfb part of https://github.com/libsdl-org/SDL/issues/3930 X11 is still todo