small fix for 64-bit Windows build

[Allin C. <cot...@wf...>]

In current CVS (on the 4.6 branch at least) the initial gnuplot screen 
shows the "Build System" as "MS-Windows 32 bit" for a Windows build 
regardless of the actual word length. The attached small patch to 
src/syscfg.h fixes this.

-- 
Allin Cottrell
Department of Economics
Wake Forest University, NC