[gtk/matthiasc/for-master] docs: Mention cairo-gobject as build requires

[Matthias Clasen <matthiasc src gnome org>]

commit 0530aeec178fbf4522150a8b764349abd15dc9c8
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Nov 16 09:29:10 2020 -0500

    docs: Mention cairo-gobject as build requires
    
    Using cairo from a system dependency and cairo-gobject
    as subproject is problematic and left me stuck on a
    hard-to-understand build error.

 docs/reference/gtk/building.md | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/docs/reference/gtk/building.md b/docs/reference/gtk/building.md
index 9092d10d1d..2ee660fb05 100644
--- a/docs/reference/gtk/building.md
+++ b/docs/reference/gtk/building.md
@@ -159,7 +159,8 @@ Other libraries are maintained separately.
   fonts and matching them against font names.
 - [Cairo](https://www.cairographics.org) is a graphics library that
   supports vector graphics and image compositing. Both Pango and GTK
-  use Cairo for drawing.
+  use Cairo for drawing. Note that we also need the auxiliary cairo-gobject
+  library.
 - [libepoxy](https://github.com/anholt/libepoxy) is a library that
   abstracts the differences between different OpenGL libraries. GTK
   uses it for cross-platform GL support and for its own drawing.