add Environment#dump

spec/build_tests_spec.rb

@@ -721,4 +721,15 @@ EOF
     expect(`./two_sources`).to eq "This is a C program with two sources.\n"
   end
 
+  it "supports dumping an Environment's construction variables" do
+    test_dir("simple")
+    env = Rscons::Environment.new do |env|
+      env["CFLAGS"] += %w[-O2 -fomit-frame-pointer]
+    end
+    env.dump
+    result = lines
+    expect(result.include?(%{CFLAGS => ["-O2", "-fomit-frame-pointer"]})).to be_truthy
+    expect(result.include?(%{CPPPATH => []})).to be_truthy
+  end
+
 end