Don't link to proc_macro on musl

This is only necessary until rust-lang/rust#49219 hits stable.

build.rs

@@ -39,7 +39,7 @@ fn main() {
 
 fn enable_use_proc_macro(target: &str) -> bool {
     // wasm targets don't have the `proc_macro` crate, disable this feature.
-    if target.contains("wasm32") {
+    if target.contains("wasm32") || target.contains("musl") {
         return false;
     }