Add a TODO for CMake 3.15.

src/CMakeLists.txt

@@ -23,7 +23,8 @@
 #   DEALINGS IN THE SOFTWARE.
 #
 
-# On MSVC remove /W3, as we are replacing it with /W4
+# On MSVC remove /W3, as we are replacing it with /W4. Could be removed as of
+# 3.15 with this: https://cmake.org/cmake/help/latest/policy/CMP0092.html
 if(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC" OR CMAKE_CXX_SIMULATE_ID STREQUAL "MSVC")
     string(REPLACE "/W3" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
 endif()