Add ability to add or replace variables

[lukaslueg]

One cannot retrieve git information if, for some reason, you are stuck with a non-regular clone. In my case, I have access to the full repository but since I want to build a subdirectory (monorepo) in docker, I lose my git context (which is completely reasonable).

In this case, I want to be able to provide the "lost" information manually, e.g. via an environment variable or by calling the write_variable macro myself in build.rs. This seems like a feature which would help alleviate the OP's issue as well.

I'm therefore suggesting two distinct features:

1. The ability to include certain environment variables in built.rs. This could be done by adding variables with a BUILT_ prefix` .
2. The ability to add or replace variables from build.rs. This could e.g. take the form of a macro set_variable!("GIT_COMMIT_HASH_SHORT", "Option<&str>", "abc1234").

If 2. is implemented, 1. probably isn't necessary as developers could easily implement the functionality themselves.

Originally posted by @FelixZY in #38 (comment)