Licensing Policy: other build files should follow cmake rule

[Harald Sitter]

Ahoy ahoy

I am proposing that we clarify the licensing policy a bit by adding a
new point following cmake:

> #. Build-related files (build scripts, makefiles, build systems other than cmake, …) should follow the cmake licensing rule. Use a permissive license, so code can easily be shared.

This is mostly to codify the intent behind the cmake rule and more
broadly cover other build systems: we want to have a permissive
license so code can be easily picked into other projects but at the
same time leave enough wiggle room so we can use other licenses here
e.g. if in a build system ecosystem the license of choice is MIT then
we still want to be free to use MIT instead.

I also considered changing around the cmake rule but I think we
benefit from its assertiveness and clearity, with cmake being the
dominant build system.

Any objections?

HS