Attempt to use ar command for static libraries when possible (#3548)

* Attempt to use `ar` command for static libraries when possible
* Fix ar invocation withour MRI script
* Correctly pass linker flags

(cherry picked from commit 9b1b616)

tools/src/main/scala/scala/scalanative/build/Discover.scala

@@ -158,14 +158,23 @@ object Discover {
   private[scalanative] val docSetup =
     "http://www.scala-native.org/en/latest/user/setup.html"
 
+  private[scalanative] def tryDiscover(
+      binaryName: String,
+      envPath: String
+  ): Try[Path] = Try(discover(binaryName, envPath))
+
+  private[scalanative] def tryDiscover(
+      binaryName: String
+  ): Try[Path] = Try(discover(binaryName))
+
   /** Discover the binary path using environment variables or the command from
    *  the path.
    */
   private[scalanative] def discover(
       binaryName: String,
-      envPath: String
+      envPath: Option[String]
   ): Path = {
-    val binPath = sys.env.get(envPath)
+    val binPath = envPath.flatMap(sys.env.get(_))
 
     val command: Seq[String] = {
       if (Platform.isWindows) {
@@ -185,14 +194,23 @@ object Discover {
       .map { p => Paths.get(p) }
       .headOption
       .getOrElse {
+        val envMessage = envPath
+          .map(envPath => s"or via '$envPath' environment variable")
+          .getOrElse("")
         throw new BuildException(
-          s"""'$binaryName' not found in PATH or via '$envPath' environment variable.
+          s"""'$binaryName' not found in PATH$envMessage.
             |Please refer to ($docSetup)""".stripMargin
         )
       }
     path
   }
 
+  private[scalanative] def discover(binaryName: String, envPath: String): Path =
+    discover(binaryName, Some(envPath))
+
+  private[scalanative] def discover(binaryName: String): Path =
+    discover(binaryName, None)
+
   /** Detect the target architecture.
    *
    *  @param clang

tools/src/main/scala/scala/scalanative/build/LLVM.scala

@@ -180,9 +180,9 @@ private[scalanative] object LLVM {
         val artifactName = outpath.getFileName().toString
         if (config.compilerConfig.buildTarget == BuildTarget.LibraryDynamic)
           if (config.targetsLinux)
-            List("-Wl,-soname", artifactName)
+            List(s"-Wl,-soname,$artifactName")
           else if (config.targetsMac)
-            List("-Wl,-install_name", artifactName)
+            List(s"-Wl,-install_name,$artifactName")
           else Nil
         else Nil
       }
@@ -220,29 +220,51 @@ private[scalanative] object LLVM {
       outpath: Path
   )(implicit config: Config) = {
     val workdir = config.workdir
-    val llvmAR = Discover.discover("llvm-ar", "LLVM_BIN")
-    val MIRScriptFile = workdir.resolve("MIRScript").toFile
-    val pw = new PrintWriter(MIRScriptFile)
-    try {
-      pw.println(s"CREATE ${escapeWhitespaces(outpath.abs)}")
-      objectPaths.foreach { path =>
+
+    val MRICompatibleAR =
+      Discover.tryDiscover("llvm-ar", "LLVM_BIN").toOption orElse
+        // MacOS ar command does not support -M flag...
+        Discover.tryDiscover("ar").toOption.filter(_ => config.targetsLinux)
+
+    def stageFiles(): Seq[String] = {
+      objectPaths.map { path =>
         val uniqueName =
           workdir
             .relativize(path)
             .toString()
             .replace(File.separator, "_")
         val newPath = workdir.resolve(uniqueName)
         Files.move(path, newPath, StandardCopyOption.REPLACE_EXISTING)
-        pw.println(s"ADDMOD ${escapeWhitespaces(newPath.abs)}")
+        newPath.abs
       }
-      pw.println("SAVE")
-      pw.println("END")
-    } finally pw.close()
+    }
 
-    val command = Seq(llvmAR.abs, "-M")
-    config.logger.running(command)
+    def useMRIScript(ar: Path) = {
+      val MIRScriptFile = workdir.resolve("MIRScript").toFile
+      val pw = new PrintWriter(MIRScriptFile)
+      try {
+        pw.println(s"CREATE ${escapeWhitespaces(config.buildPath.abs)}")
+        stageFiles().foreach { path =>
+          pw.println(s"ADDMOD ${escapeWhitespaces(path)}")
+        }
+        pw.println("SAVE")
+        pw.println("END")
+      } finally pw.close()
+
+      val command = Seq(ar.abs, "-M")
+      config.logger.running(command)
 
-    Process(command, config.workdir.toFile()) #< MIRScriptFile
+      Process(command, config.workdir.toFile()) #< MIRScriptFile
+    }
+
+    MRICompatibleAR match {
+      case None =>
+        val ar = Discover.discover("ar")
+        val command = Seq(ar.abs, "rc", config.buildPath.abs) ++ stageFiles()
+        config.logger.running(command)
+        Process(command, config.workdir.toFile())
+      case Some(path) => useMRIScript(path)
+    }
   }
 
   /** Checks the input timestamp to see if the file needs compiling. The call to