Aside from stopping the compiler error by disabling ASSIMP_TOOLS, you may still want to build the assimp viewer as it's very useful for debugging. Even with DirectX installed, assimp may block the tool from being compiled. If you believe DirectX is properly installed, you can delete the check for DirectX; the way assimp checks for DirectX seems to be wrong. Removing the BUILD_ASSIMP_VIEW setting based on if "FIND_PACKAGE" returns true allows me to build assimp viewer

https://github.com/assimp/assimp/blob/ce0a50ec21bfa4bb2535a4fbd87b67feadc0cea0/CMakeLists.txt#L753-L758

I created a GitHub issue as well https://github.com/assimp/assimp/issues/5820