Get system menu width in pixels using winapi

Question

I need to obtain the total width in pixels of the window/system menu (minimize, maximize and close) in C using basic WINAPI. I came across the WM_GETTITLEBARINFOEX message which is good for me, but unfortunately it is supported starting with Vista and my application has to support also XP and 2003. Can anyone tell me how I can make this work? Any suggestions are welcome.

Thanks in advance!