bug#49722: 27.2; Emacs won't build with old versions of fontconfig

[Eli Zaretskii]

> From: Andrei Kuznetsov <r12451428287@163.com>
> Cc: 49722@debbugs.gnu.org
> Date: Sun, 25 Jul 2021 08:53:59 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Thanks, but please show the error messages you saw before applying
> > that patch.
> 
> xftfont.c: In function ‘xftfont_cached_font_ok’:
> xftfont.c:591: error: ‘FC_LCD_FILTER’ undeclared (first use in this function)
> xftfont.c:591: error: (Each undeclared identifier is reported only once
> xftfont.c:591: error: for each function it appears in.)
> 
> It appears that FC_LCD_FILTER is not present on these older versions of
> fontconfig.

Thanks.  We once had the following in xftfont.c:

  #ifndef FC_LCD_FILTER
    /* Older fontconfig versions don't have FC_LCD_FILTER. */
  #define FC_LCD_FILTER "lcdfilter"
  #endif

But it got moved to ftfont.c when a function which used it was moved
there.  However, since xftfont.c still uses this, I think we should
have this #define in xftfont.c as well, so please try adding that at
the beginning of xftfont.c, and see if that helps.