bug#30063: 26.0.90; Silent fail with `rst-compile-pdf-preview'

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@users.sourceforge.net>
> Date: Tue, 30 Jan 2018 09:30:30 -0500
> Cc: Simen Heggestøyl <simenheg@gmail.com>, 
>       stefan@merten-home.de, 30063@debbugs.gnu.org
> 
> > Also please note that the shell command as written is unportable: the
> > ";" part will not work on MS-Windows, we need to use "&" instead.  But
> > that's a separate issue.
> 
> "&" would do the wrong thing for a POSIX sh compatible shell

I meant to leave ";" on Posix, and use "&" on Windows.