Recompile when config.yml changes

[eregon]

• Fixes Extension not fully rebuild when config.yml changes #3931.
• The first line is the default one, we want to keep that default dependency too.
• Some docs about this: https://docs.ruby-lang.org/en/4.0/MakeMakefile.html

[Earlopain]

Ah, right:

If “depend” file exist in the source directory, that content will be included in the generated makefile, with formatted by depend_rules method.

So quite easy actually. But shouldn't this include all the headers? This basically just takes care of all the gitignored/generated ones but what if for example defines.h changes?

[Earlopain]

I had unknown symbol errors after #3964 was merged so without looking too much into it I guess my comment is somewhat relevant. At least it failed loudly this time 😅

[eregon]

So quite easy actually. But shouldn't this include all the headers? This basically just takes care of all the gitignored/generated ones but what if for example defines.h changes?

Yes, I think you're right, I'll try it.

[Earlopain]

I found that adding the headers to $headers(I think) in extconf seemed to work. But I barely found any usage of that so not sure about that.

Would be nice since they can just be globbed in ruby

[eregon]

Yes, I came to the same conclusion, #3967
And those headers are included in ruby_headers so there is no need to depend on LOCAL_HDRS explicitly (but we can for clarity if wanted)