add multi-threading build test

build_tests/simple/threading.rb

@@ -0,0 +1,28 @@
+class ThreadedTestBuilder < Rscons::Builder
+  def run(options)
+    command = ["ruby", "-e", %[sleep 1]]
+    Rscons::ThreadedCommand.new(
+      command,
+      short_description: "ThreadedTestBuilder #{options[:target]}")
+  end
+  def finalize(options)
+    true
+  end
+end
+
+class NonThreadedTestBuilder < Rscons::Builder
+  def run(options)
+    puts "NonThreadedTestBuilder #{options[:target]}"
+    sleep 1
+    options[:target]
+  end
+end
+
+Rscons::Environment.new do |env|
+  env.add_builder(ThreadedTestBuilder.new)
+  env.add_builder(NonThreadedTestBuilder.new)
+  env.ThreadedTestBuilder("a")
+  env.ThreadedTestBuilder("b")
+  env.ThreadedTestBuilder("c")
+  env.NonThreadedTestBuilder("d")
+end