Improve ROOT’s build system

Header completeness

Any header should be self-contained: one should be able to #include it without #including anything else before, Sometimes this slips through, and we end up with a header that can only be compiled in some context. It would be great to have a new build target (likely per module) that compiles a generated .cxx file which only #includes one header, nothing else. We could run that target as part of our test suite or nightly builds.

Expected Results

• Standalone headers: better code!

Requirements

Basic C++ knowledge.