If the repo submodule appears to be empty or out of date, you may need to run git submodule update --init path/to/repo.
The steps to get the transpiled code are as follows:
$ intercept-build make
$ c2rust transpile compile_commands.json
$ rustc grabc.rs -L/usr/X11R6/lib -lX11
If you want to have the transpiler create a crate:
$ intercept-build make
$ c2rust transpile compile_commands.json --emit-build-files -m grabc --output-dir rust
$ cd rust
$ RUSTFLAGS="-L/usr/X11R6/lib -lX11" cargo build