Vendor dependencies

Let's see how I like this workflow.

vendor/procfs/build.rs

@@ -0,0 +1,9 @@
+fn main() {
+    // Filters are extracted from `libc` filters
+    let target_os = std::env::var("CARGO_CFG_TARGET_OS").expect("Missing CARGO_CFG_TARGET_OS envvar");
+    if !["android", "linux", "l4re"].contains(&target_os.as_str()) {
+        eprintln!("Building procfs on an for a unsupported platform. Currently only linux and android are supported");
+        eprintln!("(Your current target_os is {})", target_os);
+        std::process::exit(1)
+    }
+}