8309287: Add fontconfig requirement to building.md for Debian

Reviewed-by: erikj

Author: Jan Kratochvil

doc/building.html

@@ -685,6 +685,8 @@ <h3 id="fontconfig">Fontconfig</h3>
 Fontconfig</a> is required on all platforms except Windows and
 macOS.</p>
 <ul>
+<li>To install on an apt-based Linux, try running
+<code>sudo apt-get install     libfontconfig-dev</code>.</li>
 <li>To install on an rpm-based Linux, try running
 <code>sudo yum install     fontconfig-devel</code>.</li>
 </ul>

doc/building.md

@@ -480,6 +480,8 @@ if `configure` does not automatically locate the platform FreeType files.
 Fontconfig from [freedesktop.org Fontconfig](http://fontconfig.org) is required
 on all platforms except Windows and macOS.
 
+  * To install on an apt-based Linux, try running `sudo apt-get install
+    libfontconfig-dev`.
   * To install on an rpm-based Linux, try running `sudo yum install
     fontconfig-devel`.