capnpc: add option to build capnp exe from src

[rileylyman]

Related to #182.

This is one potential way we could implement option 2.) while keeping in mind @zenhack's concern about pushing users away from their package manager. The idea is that we check for the existence of capnp at build time. If it does not exist, we throw an error letting the user know to either install it OR provide feature = "build-capnp" to the crate so that we can build it for them.

I think that it is good to at least provide this option to the user since many may find it desirable to have a plug-and-play solution without the need for prerequisites when building their crate.

You may find adding the capnproto submodule undesirable, in which case we could potentially clone the repo at build time.

[zenhack]

I like this approach.

[Bromeon]

Randomly stumbled upon this PR and realized it has already been two years since the issue!
Very nice to see concrete ideas 🙂

[Bromeon]

Probably a matter of style, but you could just use:

PathBuf::from(env!("OUT_DIR"))
    .join("bin")
    .join("capnp")

[Bromeon]

cmake should probably be marked optional, while activated by build-capnp feature?

[features]
build-capnp = ["dep:cmake"]

[build-dependencies]
cmake = { version = "0.1", optional = true }
which = "4.2"

See also Cargo reference. The dep: syntax requires Rust 1.60.

[Bromeon]

if !which::which("capnp").is_ok() {
    panic!(msg);
}

could be written as:

which::which("capnp").expect(msg);