Cargo Features

Chords has no features set by default.

[dependencies]
chords = { version = "0.2.7", features = ["ffi_c"] }
ffi_c = libc

Features from optional dependencies

In crates that don't use the dep: syntax, optional dependencies automatically become Cargo features. These features may have been created by mistake, and this functionality may be removed in the future.

libc ffi_c?