Disallow building GPU support on Windows. (Temporarily, hopefully.)

(Related, issues #96, #89, #61, #48, and #41.)

CMakeLists.txt

@@ -152,6 +152,8 @@ if (CMAKE_CUDA_COMPILER)
 
     if ("${PBRT_OPTIX7_PATH}" STREQUAL "")
         message (WARNING "Found CUDA but PBRT_OPTIX7_PATH is not set. Disabling GPU compilation.")
+    elseif (CMAKE_SYSTEM_NAME STREQUAL Windows)
+        message (FATAL_ERROR "pbrt on the GPU is not currently supported on Windows--sorry! It has worked in the past, but is now buggy. It will be necessary for someone to debug the current issues and send a PR for it to be re-enabled (we don't currently have the bandwidth to do so).")
     else ()
         enable_language (CUDA)
         list (APPEND PBRT_DEFINITIONS "PBRT_BUILD_GPU_RENDERER")