Makefile.PL Add Win32::Process as pre-req (for Win32 only)

Have rewritten PLplot.pm's sub check() to use Win32::Process
on Windows. (Couldn't get it to do the right thing any other
way.)
lib/PDL/Graphics/Simple/PLplot.pm - use Win32::Process on Windows

The check() sub now uses Win32::Process (on MS Windows only).