feat: compile-time fallback include path

[komar007]

This is a result of a small investigation into how to make protols work out of the box with the well-known protobuf types, even though it does not ship them. See NixOS/nixpkgs#465302.

The idea is to be able to configure, in compile time, a lowest-priority include path that a distro package could set to make sure, without compromising other sources of include paths, that protols can find the well-known types even when pkg-config and protobuf are missing from the environment and no paths have been manually configured via CLI or settings.

Please advise on the best name for the environment variable.

[coder3101]

I like the idea overall. Could you also document this change in README, env variable is fine whatever the nix systems use.

[komar007]

I like the idea overall. Could you also document this change in README, env variable is fine whatever the nix systems use.

Thanks, I'll update the README.

As for the variable, there is no particular name in nix, I'll provide there whatever we settle upon in protols.

[asharkhan3101]

/lgtm