Place source and build products in OUT_DIR

This has two benefits:

* It follows the directions specified in the Cargo docs. ("Build
  scripts may save any output files in the directory specified in the
  `OUT_DIR` environment variable. Scripts should not modify any files
  outside of that directory.")

* It makes `cargo clean` work properly. (When build products are
  placed outside of `OUT_DIR`, `cargo clean` cannot remove them, which
  can make issues very difficult to debug.)

Author: jturner314

build.rs

@@ -32,7 +32,7 @@ fn main() {
             _ => variable!("TARGET"),
         };
         env::remove_var("TARGET");
-        let source = PathBuf::from(format!("source_{}", target.to_lowercase()));
+        let source = output.join(format!("source_{}", target.to_lowercase()));
         if !source.exists() {
             let source_tmp = PathBuf::from(format!("{}_tmp", source.display()));
             if source_tmp.exists() {