using bgd.dll on windows

[Bastian M. <bma...@we...>]

While trying to compile gnuplot on Windows using MSVC 6.0
and the precompiled gdlib (gdwin32.zip package) I ran into
problems concerning calling convention. The supplied
bgd.dll now uses __stdcall whereas gnuplot uses __cdecl.

The attached patch fixes this issue. Could you please check
if it doesn't break anything?