Add an option to output a depfile for outside build-systems to learn
the source file dependencies of the bindings.
This can be used by 3rd party build system integrations to only rerun
bindgen when necessary.
Testing is done via CMake integration tests, since CMake
is a 3rd party buildsystem which supports depfiles.
Since our MSRV is now Rust 1.54, we can rely on cargo setting `CARGO_BIN_EXE_cbindgen`
to the cbindgen path in integration tests.
This is more reliable than guessing the path, since cargo knows where it placed the bin.
