Don't use the std feature of libc.

It's not needed, and prevents some bindgen users from being able to run bindgen
at build time in no_std crates, see:

  rust-lang/rust-bindgen#1439

Cargo.toml

@@ -43,7 +43,7 @@ static = []
 [dependencies]
 
 glob = "0.2.11"
-libc = "0.2.39"
+libc = { version = "0.2.39", default-features = false }
 libloading = { version = "0.5.0", optional = true }
 
 [build-dependencies]