closes https://github.com/assimp/assimp/issues/3004 : use prefix when ms_tools were found.

CMakeLists.txt

@@ -684,7 +684,8 @@ if(WIN32)
   ENDIF()
 
   IF(MSVC_TOOLSET_VERSION)
-    set(MSVC_PREFIX "vc${MSVC_TOOLSET_VERSION}")
+    SET(MSVC_PREFIX "vc${MSVC_TOOLSET_VERSION}")
+    SET(ASSIMP_MSVC_VERSION ${MCVS_PREFIX})
   ELSE()
     IF(MSVC12)
       SET(ASSIMP_MSVC_VERSION "vc120")