ExternalProject.cmake raises CMP0054 warning, failing Appveyor builds
Appveyor uses CMake 3.9.1. When building for VS2017, we get a CMP0054 warning at C:/Program Files (x86)/CMake/share/cmake-3.9/Modules/ExternalProject.cmake:1954. This fails the appveyor build as it causes a powershell exception. Tested on 3.9.4, still an issue. This does not happen on VS2015 builds.
The project builds successfully, but we cannot "pass" until the CMP0054 policy is set to prevent this warning.