Vendor dependencies

Let's see how I like this workflow.

vendor/signal-hook/build.rs

@@ -0,0 +1,9 @@
+#[cfg(feature = "extended-siginfo-raw")]
+fn main() {
+    cc::Build::new()
+        .file("src/low_level/extract.c")
+        .compile("extract");
+}
+
+#[cfg(not(feature = "extended-siginfo-raw"))]
+fn main() {}