[cmake] Diagnose missing header (and include path) for externally built libraries.

[vgvassilev]

ROOT has several externally built packages which end up in the dictionaries, such as Vc.

Currently, if the Vc is externally built, the MathCore dictionary needs to know its include location at rootcling-time but also when running ROOT (ROOT's runtime). If the person uses that approach does not pass this information, this leads to very subtle crash in modules and it takes time to setup a debug build and diagnose.

Instead, the dictionary which has an include to an externally built library (in ROOT_GENERATE_DICTIONARY) should be able to check if this include path is also passed in ROOT for its runtime. Otherwise, issue a diagnostic.

[ferdymercury]

tagging @guitargeek @hageboeck since they have been playing with runpaths and external libs.

(Vc is deprecated so maybe no longer very important)

[guitargeek]

This issue still affects vdt, but I'm proposing to avoid including vdt in the modules to fix that:

• [RVec] Forward declare VDT functions in RVec.hxx header #22927