Add flag-sensitive build job helper for FFI code

[arthurpaulino]

I was trying to build a project that uses the FFI and there was a compilation error telling me to add -fPIC as a parameter. I tried to add it in many ways, including the right one: in the array of flags. Either way didn't work.

Turns out I just had to do lake clean before trying lake build again, which is a bit strange because I was expecting lake build to notice that I had a different build setup.

For more context: https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/C.20FFI.20usage/near/299865961

[tydeu]

This is because your ffi.o is a custom target. In order to make it rebuild upon the changing of compile flags you need to include those flags in the trace of that build (e.g., via extraDepTrace on buildAfterDep). The builtin targets for module compilation already do this (see buildLeanO).

[Kha]

Reinterpreting this issue as a feature request, I suppose a generic buildO job capturing this common pattern would be a nice addition