I could not build first SlicerRT then SlicerOpenIGTLink today, and it seemed that the reason was a custom Python being installed on my Windows machine. See Plastimatch build fails · Issue #198 · SlicerRt/SlicerRT · GitHub
After chatting with @lassoan we decided that we define the missing (and then after uninstalling the custom Python undefined) Python variables, see COMP: Fix Plastimatch configuration if Python was installed · SlicerRt/SlicerRT@8b67847 · GitHub and COMP: Fix subproject configuration if Python was installed by cpinter · Pull Request #116 · openigtlink/SlicerOpenIGTLink · GitHub .
My question is, mainly to @jcfr if there is a better solution for this, other than manually adding these variables to the subproject CMake files that fail to configure in this case. Thanks!