fix toolset for boost

packages/b/boost/xmake.lua

@@ -42,6 +42,8 @@ package("boost")
         if file then
             if is_plat("macosx") then
                 file:print("using darwin : : %s ;", package:build_getenv("cxx"))
+            elseif is_plat("windows") then
+                file:print("using msvc : : %s ;", package:build_getenv("cxx"))
             else
                 file:print("using gcc : : %s ;", package:build_getenv("cxx"))
             end