Index: cmake/Modules/CompilerRTCompile.cmake
===================================================================
--- cmake/Modules/CompilerRTCompile.cmake
+++ cmake/Modules/CompilerRTCompile.cmake
@@ -21,9 +21,13 @@
     string(REPLACE " " ";" global_flags "${CMAKE_C_FLAGS}")
   endif()
   # On Windows, CMAKE_*_FLAGS are built for MSVC but we use the GCC clang.exe
-  # which doesn't support flags starting with "/smth". Replace those with
-  # "-smth" equivalents.
+  # so perform any required transformations to ensure that they are accepted.
   if(MSVC)
+    # The MSVC warning suppression/promotion flags are meaningless and
+    # unsupported here so remove them.
+    string(REGEX REPLACE "/w[de]?[0-9]+(;|$)" "" global_flags "${global_flags}")
+
+    # Replace flags starting with "/smth" with "-smth" equivalents.
     string(REGEX REPLACE "^/" "-" global_flags "${global_flags}")
     string(REPLACE ";/" ";-" global_flags "${global_flags}")
   endif()