diff --git a/third_party/meshoptimizer/src/vertexcodec.cpp b/third_party/meshoptimizer/src/vertexcodec.cpp
index a16e2b640f..8e5d96876c 100644
--- a/third_party/meshoptimizer/src/vertexcodec.cpp
+++ b/third_party/meshoptimizer/src/vertexcodec.cpp
@@ -12,7 +12,7 @@
 #define SIMD_SSE
 #endif
 
-#if !defined(SIMD_SSE) && defined(WIN32) && (defined(_M_IX86) || defined(_M_X64))
+#if !defined(SIMD_SSE) && defined(_MSC_VER) && (defined(_M_IX86) || defined(_M_X64))
 #define SIMD_SSE
 #define SIMD_FALLBACK
 #include <intrin.h>
diff --git a/third_party/meshoptimizer/tnt/README b/third_party/meshoptimizer/tnt/README
index 8d2f27ea73..b8cc754b34 100644
--- a/third_party/meshoptimizer/tnt/README
+++ b/third_party/meshoptimizer/tnt/README
@@ -8,12 +8,15 @@ This folder was created as follows:
     rm -rf demo tools
     mv LICENSE.md LICENSE
 
-Also removed lines 11 - 24 vertexcodec.cpp to disable SSE, which caused build
-issues with windows.
+Also added this to the CMakeLists.txt:
+
+    if(CLANG_CL)
+        # Clang for Windows needs an additional flag for SSSE3 instructions.
+        target_compile_options(meshoptimizer PRIVATE -mssse3)
+    endif()
 
 To update the library, you can do this:
 
     curl -L -O https://github.com/zeux/meshoptimizer/archive/master.zip
     unzip master.zip
     cp -r meshoptimizer-master/src/ meshoptimizer/src/
-    in vertexcodec.cpp, replace _MSC_VER with WIN32