bug#20984: 25.0.50; Combining accents don't display properly in certain fonts

[Artur Malabarba]

>> Should I paste the full *help* buffer here?
>
> There's no need, thanks.  But it would be interesting, I think, to see
> the output of "C-u C-x =" for the same pair of inserted characters,
> but with your default font (assuming it is capable of displaying the
> accented a).
>

With my default font (DejaVu Sans Mono), which does correctly display
the grave a, describing the combined characters gives the following

Composed with the following character(s) "̀" using this font:
  xft:-unknown-DejaVu Sans Mono-normal-normal-normal-*-15-*-*-*-m-0-iso10646-1
by these glyphs:
  [0 1 97 68 9 1 8 11 0 nil]
  [0 1 768 648 0 2 6 12 -9 [-9 0 0]]

And describing them individually gives the following

xft:-unknown-DejaVu Sans
Mono-normal-normal-normal-*-15-*-*-*-m-0-iso10646-1 (#x44)
xft:-unknown-DejaVu Sans
Mono-normal-normal-normal-*-15-*-*-*-m-0-iso10646-1 (#x288)

> Anyway, I've tried this on my system, after installing version 1.020
> of the font, and I don't see the problem you report: the 2 characters
> are composed and displayed as you'd expect.
>
> So I think this might be some issue with the shaping engine you are
> using -- do you have the latest versions of the libraries mentioned in
> INSTALL, under "Complex Text Layout support libraries"?  If you do,
> perhaps Handa-san could comment on this.

Out of the 3 mentioned there ("m17n-db", "libm17n-flt", "libotf")
- I see the following 2 installed (and up-to-date) on my system
(arch): m17n-db, m17n-lib, libotf.
- Looking through my emacs's config.log, I see reports that the
following two were detected: M17N_FLT, LIBOTF.