Warn for Use of Symbol from Indirectly Included Header/Included Header from which No Symbols are Used

[ghost]

[yes, this is a duplicate of #234 and #236. Those issues were closed due to testing on an entire codebase rather than minimal test cases first. I think it's worth giving them another shot, without that pollution.]

Consider the following files:

// a.h

int f(char a) { /* ... */ }

//b.h

#include "a.h"

// c.c

#include "b.h"

void g() {
  // ...
  int n = f(x);
  // ...
}

c.c depends indirectly on a.h. Technically sound, but a real pest for compilation. I propose a flag, -Windirect-header, to detect and warn for this.

The latter idea is self-explanatory. Unused headers may not be as big a pest for compilation, but they still clutter the code and make it harder to trace dependencies. I propose -Wunused-header for this.

---

So as not to pollute the compile log, such warnings would ideally be printed per header rather than per reference, i.e.:

WARN: File main.c depends on header aux.h via route.h [using symbols jackConnect, jackStatus]
WARN: File main.c depends on header usb.h via route.h [using symbols jackPoll, jackBroadcast]
...

A wrinkle in this idea is that many projects have "meta-headers", that is, header files that only serve to indirectly provide a lot of smaller headers in one. Indirect warnings via these, and unused warnings within them, are frivolous. A solution would be some way to mark such headers as transitive, for instance by --transitive janet.h,zmq.h, thus disabling all such warnings.