Index: cmake/Modules/CMakePolicy.cmake
===================================================================
--- cmake/Modules/CMakePolicy.cmake
+++ cmake/Modules/CMakePolicy.cmake
@@ -10,3 +10,8 @@
 if(POLICY CMP0116)
   cmake_policy(SET CMP0116 OLD)
 endif()
+# CMP0091: MSVC runtime library flags are selected by an abstraction.
+# New in CMake 3.15. https://cmake.org/cmake/help/latest/policy/CMP0091.html
+if(POLICY CMP0091)
+  cmake_policy(SET CMP0091 OLD)
+endif()