Fix msvc windows 64 build; Add windows 64 bit prebuilt lib

- GCL_HICON is not defined for win 64, replace it by GCLP_HICON to make both win 32 and 64 happy (thanks sven! )
More info here: 
https://www.mail-archive.com/plplot-general@lists.sourceforge.net/msg00984.html

- Use "flow build --arch 64 --clean" if you prefer your own clean win 64 build