Make clap optional for build scripts

[RazrFalcon]

There is no need in clap when we use cbindgen in a build script.

[eqrion]

I'd like to do this too. rust-bindgen has the same problem.

Unfortunately - rust-lang/cargo#1982

[RazrFalcon]

You can use features for this.

[eqrion]

Was able to find a potential workaround here for the cargo issue.

[[bin]]
name = "mybinary"
path = "src/main.rs"
required-features = ["binaries"]

[dependencies]
docopt = { version = "0.8.1", optional = true }

[features]
binaries = ["docopt"]

Unfortunately that would require all users to install the cbindgen binary with cargo install cbindgen --features=binary or else they'll get a 'descriptive' error. This could be okay.

[val-ms]

Looks like this was already done, here: 6748b78

And you can disable building with clap by disabling the default features. E.g. this in your own Cargo.toml:
cbindgen = { version ="0.25", default-features = false }