CMake: No Deprecation Warnings (#321)

Setting a `cmake_policy` to `OLD` is deprecated by definition and
will raise a verbose warning.

CMakeLists.txt

@@ -16,6 +16,12 @@ endif()
 
 # CMake policies ##############################################################
 #
+# Setting a cmake_policy to OLD is deprecated by definition and will raise a
+# verbose warning
+if(CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR)
+    set(CMAKE_WARN_DEPRECATED OFF CACHE BOOL "" FORCE)
+endif()
+
 # CMake 3.18+: CMAKE_CUDA_ARCHITECTURES
 # https://cmake.org/cmake/help/latest/policy/CMP0104.html
 if(POLICY CMP0104)