Some unicode symbols can't be displayed

[Julien Cubizolles <j.cubizolles@free.fr>]

Some unicode symbol aren't properly displayed in gtk emacs -Q. I only get
a square block with the unicode number in it.

Here is an example (output of describe-char):

--8<---------------cut here---------------start------------->8---
             position: 15833 of 27566 (57%), column: 25
            character: 👍 (displayed as 👍) (codepoint 128077, #o372115, #x1f44d)
    preferred charset: unicode (Unicode (ISO10646))
code point in charset: 0x1F44D
               script: symbol
               syntax: w 	which means: word
             category: .:Base
             to input: type "C-x 8 RET 1f44d" or "C-x 8 RET THUMBS UP SIGN"
          buffer code: #xF0 #x9F #x91 #x8D
            file code: #xF0 #x9F #x91 #x8D (encoded by coding system utf-8-unix)
              display: no font available

Character code properties: customize what to show
  name: THUMBS UP SIGN
  general-category: So (Symbol, Other)
  decomposition: (128077) ('👍')

There are text properties here:
  fontified            t
--8<---------------cut here---------------end--------------->8---

What's even more confusing: it's correctly displayed as a thumbs up sign
in emacs -nw, and on another machine where I have basically the same
emacs setup.

I'm running GNU Emacs 26.0.50 (build 2, x86_64-pc-linux-gnu, GTK+
Version 3.20.9) of 2017-04-04

Julien.