add backwards-compatibility test for Builder#standard_build

build_tests/simple/standard_build.rb

@@ -0,0 +1,17 @@
+class MyCommand < Rscons::Builder
+  def run(target, sources, cache, env, vars)
+    vars = vars.merge({
+      "_TARGET" => target,
+      "_SOURCES" => sources,
+    })
+    command = env.build_command("${CMD}", vars)
+    cmd_desc = vars["CMD_DESC"] || "MyCommand"
+    standard_build("#{cmd_desc} #{target}", target, command, sources, env, cache)
+  end
+end
+
+Rscons::Environment.new do |env|
+  env.add_builder(MyCommand.new)
+  command = %w[gcc -c -o ${_TARGET} ${_SOURCES}]
+  env.MyCommand("simple.o", "simple.c", "CMD" => command)
+end