fix cmake build on msvc

Author: Patrick

CMakeLists.txt

@@ -252,7 +252,7 @@ endif()
 if (NOT MSVC)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fpermissive -std=c++11")
 else()
-    set(CMAKE_CXX_FLAGS "$(CMAKE_CXX_FLAGS) /EHsc /MP")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /EHsc /MP")
 endif()
 
 if (ANVIL_LINK_WITH_GLSLANG)