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.

diff --git a/docs/reference/gtk/building.md b/docs/reference/gtk/building.md
index 9092d10d1d8c89d84c30d9a176113738d021fb91..2ee660fb0555d47f232b6f1377f39034f6610d92 100644 (file)
--- 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.