Check for not clang when checking for MSVC

Task-number: QTBUG-63512
Change-Id: I9cd97dc178c108dfca0b1ee12dd18b78497603d7
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/3rdparty/libwebp/src/dsp/dsp.h

@@ -48,7 +48,8 @@ extern "C" {
 #endif
 
 #if defined(_MSC_VER) && _MSC_VER >= 1500 && \
-    (defined(_M_X64) || defined(_M_IX86))
+    (defined(_M_X64) || defined(_M_IX86)) && \
+    !defined(__clang__)
 #define WEBP_MSC_SSE41  // Visual C++ SSE4.1 targets
 #endif