Build.cs variable name for visual studio installation

I have a plugin project that makes use of the Microsoft MFC classes. In the build.cs file I added an include path (PublicIncludePaths), to the MFC header directory and also a reference (PublicAdditionalLibraries) to the Microsoft library that is needed for the compile. For my local compiles I just used a hardcoded path for these entries but now that I want to submit it to the Marketplace I need a way to reference the MFC classes wherever it might be installed.

Can someone tell me what variable(s) might be available that I can use in build.cs in order to turn the absolute paths into relative paths? How can I use these variables to reference the locations that I need?