VS2017 solution attempts to open openframeworksLib project from wrong drive

I just downloaded a fresh openframeworks build and placed it on a secondary drive at D:\openFrameworks.

My repository wason the C: drive, on the Desktop. I then used the projectGenerator to generate a VS solution. When I open it I get the following errors:

\Users\sol\Desktop\VJmidiKit\VJmidiKit.vcxproj : error  : The imported project "C:\openFrameworks\libs\openFrameworksCompiled\project\vs\openFrameworksDebug.props" was not found. Confirm that the path in the <Import> declaration is correct, and that the file exists on disk.  C:\Users\sol\Desktop\VJmidiKit\VJmidiKit.vcxproj

C:\openFrameworks\libs\openFrameworksCompiled\project\vs\openframeworksLib.vcxproj : error  : The project file could not be loaded. Could not find a part of the path 'C:\openFrameworks\libs\openFrameworksCompiled\project\vs\openframeworksLib.vcxproj'.  C:\openFrameworks\libs\openFrameworksCompiled\project\vs\openframeworksLib.vcxproj

As you can see, VS seems to confuse OF_ROOT and look in C:\openFrameworks. I went back to the projectGenerator to verify it but it is set correctly (I also tried regenerating the project):

In the generated .vcxproj I see these two declarations involving paths:

<ProjectReference Include="$(OF_ROOT)\libs\openFrameworksCompiled\project\vs\openframeworksLib.vcxproj">
  <AdditionalIncludeDirectories>$(OF_ROOT)\libs\openFrameworksCompiled\project\vs</AdditionalIncludeDirectories>

However I cannot find any reference to OF_ROOT (except comments in my unix Makefile). Where is it set from?

Start with the projectGenerator output and the generated VJmidiKit.vcxproj, then trace how the $(OF_ROOT) ProjectReference and AdditionalIncludeDirectories values are supplied. Done means a build generated from D:\openFrameworks no longer resolves openFrameworksDebug.props or openframeworksLib.vcxproj under C:\openFrameworks.