Vendor dependencies

Let's see how I like this workflow.

vendor/log/build.rs

@@ -0,0 +1,46 @@
+//! This build script detects target platforms that lack proper support for
+//! atomics and sets `cfg` flags accordingly.
+
+use std::env;
+use std::str;
+
+fn main() {
+    let target = match rustc_target() {
+        Some(target) => target,
+        None => return,
+    };
+
+    if target_has_atomic_cas(&target) {
+        println!("cargo:rustc-cfg=atomic_cas");
+    }
+
+    if target_has_atomics(&target) {
+        println!("cargo:rustc-cfg=has_atomics");
+    }
+
+    println!("cargo:rerun-if-changed=build.rs");
+}
+
+fn target_has_atomic_cas(target: &str) -> bool {
+    match &target[..] {
+        "thumbv6m-none-eabi"
+        | "msp430-none-elf"
+        | "riscv32i-unknown-none-elf"
+        | "riscv32imc-unknown-none-elf" => false,
+        _ => true,
+    }
+}
+
+fn target_has_atomics(target: &str) -> bool {
+    match &target[..] {
+        "thumbv4t-none-eabi"
+        | "msp430-none-elf"
+        | "riscv32i-unknown-none-elf"
+        | "riscv32imc-unknown-none-elf" => false,
+        _ => true,
+    }
+}
+
+fn rustc_target() -> Option<String> {
+    env::var("TARGET").ok()
+}