Add -fstrict-headers

[Vexu]

Closes #234

Should this be on by default? Should it be an error or a warning? Maybe it should be a warning if it wasn't explicitly set on.

[ghost]

Great work, but just one tiny nit: indirect dependencies aren't the only way headers can be abused. I've proposed a new name (-Windirect-include) in the original issue, and also proposed another warning (-Wunused-include) for a different header abuse (sorry I'm slow 😞). Perhaps -Wstrict-headers could include both? (NOPE NOPE NOPE NOPE NOPE)

[Vexu]

I added the unused include error too but it seems like it might cause a lot of false positives. Could you test it out and see how it feels?

Also should the direct include rule apply to macros too?

[ghost]

Hooolyyyy mackerel. I just built Janet with the latest commit, and it threw 215 errors, of which only 7 were from the project itself, and maybe one was a true positive. musl isn't quite as bad on the surface, but tedious to trace.

I've changed my mind. -Wunused-include is a bad idea.

(But yes, I do think the direct include rule should apply to macros. I'm thinking of the dev who has to track down all the #defines.)

[Vexu]

@EleanorNB I re-added the unused import warning with some bug fixes and made it not apply to system headers, wanna give it another try?

[ghost]

Hm, still throwing the same errors for me, even on branch tip. Most indirect symbol warnings (couldn't count, too many) are for system headers, and some notes suggest using headers from my downloaded copy of Aro to fix them. Same story with unused.

Below is the full error log of compiling https://github.com/janet-lang/janet commit 66e0b53cf655c5981d4e9974d354e3f0240290a8 with branch tip.
https://zigbin.io/881d09

[Vexu]

Closing as this never worked in practice and isn't a high priority. Maybe someday someone will finish it.