Vendor dependencies

Let's see how I like this workflow.

vendor/libc/tests/const_fn.rs

@@ -0,0 +1,5 @@
+#![cfg(libc_const_extern_fn)] // If this does not hold, the file is empty
+
+#[cfg(target_os = "linux")]
+const _FOO: libc::c_uint = unsafe { libc::CMSG_SPACE(1) };
+//^ if CMSG_SPACE is not const, this will fail to compile