Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -200,8 +200,13 @@
 
 include(config-ix)
 
-if(MSVC)
-  append_string_if(COMPILER_RT_HAS_W3_FLAG /W3 CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+if(MSVC AND COMPILER_RT_HAS_W3_FLAG)
+  # MSVC will give a warning if there is already another warning option.
+  # Remove any such option if present before appending /W3.
+  string(REGEX REPLACE " /W[0-4]" "" CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
+  string(REGEX REPLACE " /W[0-4]" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /W3")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /W3")
 else()
   append_string_if(COMPILER_RT_HAS_WALL_FLAG -Wall CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 endif()