Embed Git metadata in C/C++ project

This is a demo project that shows how to embed up-to-date git metadata in a C/C++ project via CMake. The entire capability is baked into single self-contained script.

"The proof of the pudding is in the eating!"

In other words, please clone the repository and try out the demo.

A "why does this matter" use case

We're continuously shipping prebuilt binaries for an application. A user discovers a bug and files a bug report. By embedding up-to-date versioning information, the user can include this in their report, e.g.:

Commit SHA1: 46a396e (46a396e6c1eb3d)
Dirty: false (there were no uncommitted changes at time of build)

This allows us to investigate the precise version of the application that the bug was reported in.

Wait, doesn't this already exist?

Well, it depends on your specific requirements. Before writing this, I searched far and wide for existing solutions. Each solution I found fell into one of two categories:

Write the commit ID to the header at configure time (e.g. cmake <source_dir>). This works well for automated build processes (e.g. check-in code and build artifacts). However, it has one weakness: any changes made after running cmake (e.g. git commit -am "Changed X") aren't reflected in the header.
Every time a build is started (e.g. make), write the commit ID to a header. While this was better than the above, it had one major drawback: any object file that includes the new header will be recompiled -- even if the state of the git repo hasn't changed.

So what's the ideal solution?

We check Git every time a build is started (e.g. make) to see if anything has changed, like a new commit to the current branch. If nothing has changed, then we don't touch anything- no recompiling or linking is triggered. If something has changed, then we reconfigure the header and CMake rebuilds any downstream dependencies.

Tip: how to avoid unnecessary recompilations

If you're worried about lengthy recompilations, then don't place the versioning information in a header that is then included in every source file. Doing so would defeat the purpose of partial rebuilds.