Get all preprocessor definitions from VCProject

I have succeeded in getting some of the VCProject options in a Visual Studio Extension that I am writing. I see that I can use VCCLCompilerTool.FullIncludePath to get the set of directories used to search for header files.

I also see VCCLCompilerTool.PreprocessorDefinitions, but this only has the preprocessor definitions defined in the project, not inherited values such as UNICODE and _UNICODE.  How would I get the full set of preprocessor definitions, including inherited values?