[gtkmm-documentation] Describe how to build a multi-threaded program

[Kjell Ahlstedt <kjellahl src gnome org>]

commit f29bef60a249b81486605149cdd5bf4c05ed2e4f
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Mon Aug 17 11:13:43 2020 +0200

    Describe how to build a multi-threaded program
    
    Fixes #12

 docs/tutorial/C/index-in.docbook | 8 ++++++++
 1 file changed, 8 insertions(+)
---
diff --git a/docs/tutorial/C/index-in.docbook b/docs/tutorial/C/index-in.docbook
index f9c06f33..7492e7c0 100644
--- a/docs/tutorial/C/index-in.docbook
+++ b/docs/tutorial/C/index-in.docbook
@@ -7358,6 +7358,14 @@ by a <classname>std::mutex</classname>.
 Only the GUI thread updates the GUI.
 </para>
 
+<para>
+Compiling and linking a multi-threaded program can require special compiler and
+linker options. If you use the <application>g++</application> compiler, add the
+<literal>-pthread</literal> option. Other compilers may require other options.
+If you build with <application>meson</application>, it handles the multi-threading
+complications for you, if you add <function>dependency('threads')</function>.
+</para>
+
 <figure id="figure-multithread">
   <title>Multi-Threaded Program</title>
   <screenshot>