build.rs: Fold more work into nasm().

Prepare for removing the `compile()` function.

build.rs

@@ -532,10 +532,8 @@ fn compile(b: &mut cc::Build, p: &Path, target: &Target, include_dir: &Path, out
     if !matches!(p.extension(), Some(e) if e == "asm") {
         let _ = b.file(p);
     } else {
-        let out_file = obj_path(out_dir, p);
-        let cmd = nasm(p, &target.arch, include_dir, &out_file);
-        run_command(cmd);
-        let _ = b.object(out_file);
+        let obj = nasm(p, &target.arch, include_dir, out_dir);
+        let _ = b.object(obj);
     }
 }
 
@@ -618,7 +616,9 @@ fn configure_cc(c: &mut cc::Build, target: &Target, include_dir: &Path) {
     }
 }
 
-fn nasm(file: &Path, arch: &str, include_dir: &Path, out_file: &Path) -> Command {
+fn nasm(file: &Path, arch: &str, include_dir: &Path, out_dir: &Path) -> PathBuf {
+    let out_file = obj_path(out_dir, file);
+
     let oformat = match arch {
         "x86_64" => "win64",
         "x86" => "win32",
@@ -644,7 +644,8 @@ fn nasm(file: &Path, arch: &str, include_dir: &Path, out_file: &Path) -> Command
         .arg("-Xgnu")
         .arg("-gcv8")
         .arg(file);
-    c
+    run_command(c);
+    out_file
 }
 
 fn run_command_with_args<S>(command_name: S, args: &[String])