Add Opt-Out Flag to Disable MSBuild Automation

[josephdrake-stahls]

I'm running into issues manually calling SlowCheetah.TransformTask largely in part because there is no way to disable built in targets.

Scenario

For cleaner code repository, we don't commit a App.config or Web.config. Instead we have an App.Template.config that is used for the source file. The way to do this with SlowCheetah is like this (includes Binding Redirect generation):

	<Target Name="_XmlConfigTransform_TransformAppConfig__Before_GetCopyToOutputDirectoryItems" BeforeTargets="GetCopyToOutputDirectoryItems" Condition="Exists('App.Template.config') and Exists('App.$(Configuration).config')">
		<SlowCheetah.TransformTask
			Source="App.Template.config"
			Transform="App.$(Configuration).config"
			Destination="App.config" />
		<GenerateBindingRedirects
			AppConfigFile="App.config"
			OutputAppConfigFile="App.config"
			SuggestedRedirects="@(SuggestedBindingRedirects)" />
	</Target>

Problem

1. ScCollectAppFiles always run and tries to copy the App.config to the intermediate directory. If the App.config doesn't exists yet, it blows up.
   

   slow-cheetah/src/Microsoft.VisualStudio.SlowCheetah/Build/Microsoft.VisualStudio.SlowCheetah.App.targets

   Line 54 in d923677

   <Copy SourceFiles="$(AppConfig)" DestinationFiles="$(ScIntermediateAppConfig)"/>

2. ScTransformAppConfig always runs and regardless of any transformations occurring will replace the AppConfig property with the App.config that was copied to the intermediate directory.
   

   slow-cheetah/src/Microsoft.VisualStudio.SlowCheetah/Build/Microsoft.VisualStudio.SlowCheetah.App.targets

   Line 74 in d923677

   <AppConfig>$(ScIntermediateAppConfig)</AppConfig>

Solution

Add an "opt-out" flag that disables all SlowCheetah automation.