Fixed build warnings

src/api/caca/ffi.rs

@@ -1,6 +1,6 @@
+#![allow(non_camel_case_types)]
+
 use libc;
-use std::ffi::CStr;
-use std::mem;
 
 pub type caca_display_t = libc::c_void;
 pub type caca_canvas_t = libc::c_void;

src/api/caca/mod.rs

@@ -1,4 +1,5 @@
 #![cfg(any(target_os = "linux", target_os = "freebsd"))]
+#![allow(unused_variables, dead_code)]
 
 use libc;
 use api::osmesa::{OsMesaContext, OsMesaCreationError};