Vendor dependencies

Let's see how I like this workflow.

vendor/cxxbridge-flags/src/impl.rs

@@ -0,0 +1,20 @@
+#[allow(unused_assignments, unused_mut, unused_variables)]
+pub const STD: &str = {
+    let mut flags = ["-std=c++11", "/std:c++11"];
+
+    #[cfg(feature = "c++14")]
+    (flags = ["-std=c++14", "/std:c++14"]);
+
+    #[cfg(feature = "c++17")]
+    (flags = ["-std=c++17", "/std:c++17"]);
+
+    #[cfg(feature = "c++20")]
+    (flags = ["-std=c++20", "/std:c++20"]);
+
+    let [mut flag, msvc_flag] = flags;
+
+    #[cfg(target_env = "msvc")]
+    (flag = msvc_flag);
+
+    flag
+};

vendor/cxxbridge-flags/src/lib.rs

@@ -0,0 +1,7 @@
+//! This crate is an implementation detail of the `cxx` and `cxx-build` crates,
+//! and does not expose any public API.
+
+mod r#impl;
+
+#[doc(hidden)]
+pub use r#impl::*;