bug#36357: Wrong Ghostscript program name on MS Win

[Sebastian Urban]

> > as I found out recently, adding only gswin32c was half of the      
> > problem.  After upgrading to 64bit OS and installing Ghostscript   
> > 64bit, there was no gswin32c.exe in the "bin" folder, only         
> > gswin64.exe and gswin64c.exe.  They forgot(?) to add it to the     
> > table in "Use" documentation, i.e. "gswin32c or gswin64c", instead 
> > of only "gswin32c", although they pointed this out in "Install"    
> > documentation                                                      
> > (https://www.ghostscript.com/doc/9.51/Install.htm#General_Windows).
> > So I think another "if" is needed.                                 
>                                                                      
> What "if" did you have in mind?  Emacs cannot know which version of 
> Ghostscript is installed, and there's no way I know of to code a    
> reasonable condition for that.                                      

Whether the OS is 64bit or 32bit, or if this is not possible or
difficult, then maybe 'system-configuration' variable.  It's not
perfect, because for example someone can install 32bit GS on 64bit OS,
but most probably - on 64bit Win someone will use 64bit Emacs and
64bit GS.  This doesn't solve the problem, but better defaults are
better defaults.

As an simpler alternative, it could be changed to 'gswin64c', because
these days people are using 64bit versions usually.

> However, the variable is a defcustom, so every user can customize it
> to the value which fits their system, if the default doesn't.       

Of course, but this is plan B.