add integration test for Environment#build_sources

build_tests/simple/build_sources.rb

@@ -0,0 +1,14 @@
+class MyProgram < Rscons::Builder
+  def run(options)
+    target, sources, cache, env, vars = options.values_at(:target, :sources, :cache, :env, :vars)
+    objects = env.build_sources(sources, [".o"], cache, vars)
+    command = %W[gcc -o #{target}] + objects
+    return false unless env.execute("#{name} #{target}", command)
+    target
+  end
+end
+
+Rscons::Environment.new do |env|
+  env.add_builder(MyProgram.new)
+  env.MyProgram("simple.exe", "simple.c")
+end