Those come from Orbiters Property-Sheets (located at Orbitersdk\resources\) and should not be changed (at least not without expectig trouble ) Oh! When they come from the git repository, they seem to be wrong! There should never be an absolute path in there!
All those sheets "derive" somehow their base path (OrbiterDir) from Orbitersdk\resources\orbiterroot.props
In there it should read something like this:
In Orbiter BETA the Sheets where re-located to be at Orbitersdk\VS2015\BuildRules\ and Orbitersdk\VS2015\PropertyPages\ .
The "old" Visual Studio Propertiy Sheets (Extension .vsprops) could not handle the $(MSBuildThisFileDirectory) Variable and therefore it contained the arbitrary (in your case wrong) value "c:\source\orbiter"
So in case you have to use the old Sheets (made for Visual Studio 2010 or lower I think), you might have to change that value in there.
Ah, I see you were using Orbitersdk\samples\CustomMFD\CustomMFD.vcproj . That's unfortunately still referencing the "old" Sheets - as it's an "old" Project file.
BTW: Almost all Projects in that sample directory should have been updated to use "more modern" Sheets a long time ago.
Long story short: Either move your Orbiter development to c:\source\orbiter, or change the value in the Orbitersdk\resources\orbiterroot.vsprops file.