bug#33748: 27.0.50; harfbuzz branch fails to compile

[Eli Zaretskii]

> Date: Sat, 15 Dec 2018 08:59:43 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: behdad@behdad.org, 33748@debbugs.gnu.org, far.nasiri.m@gmail.com
> 
> Which oldest version of HarfBuzz would you prefer us to require?  Is
> 0.9.42 OK, or do we need a newer one?

Also, if we require at least 0.9.42, then the test for
hb_ft_font_create_referenced, which was introduced in 0.9.38, can be
dropped, together with the #ifdef'ed code that relies on that test,
right?