This allows expressing "use these vpaths if this configuration is present in the project", and placing the vpaths in the same configuration block as the file lists.
- Adds externalRule() to match rule()
- Adds externalProject() as replacement for external() for consistency with rules
- Adds rules() to associate rules (external or generated) with a project
- Drops customRules(); use rules() on the project level instead
- use like: filter { "configurations:Debug" }
- supported prefixes: action, architecture, configurations, files, kind, language, options, platforms, system
- defaults to "configurations" if no prefix is specified
