Add an option to make glib-compile-resources use G_GNUC_INTERNAL

https://bugzilla.gnome.org/show_bug.cgi?id=687742

docs/reference/gio/glib-compile-resources.xml

@@ -147,6 +147,19 @@ can manually call at initialization and uninitialization time.
 </para></listitem>
 </varlistentry>
 
+<varlistentry>
+<term><option>--internal</option></term>
+<listitem><para>
+By default code generated by <option>--generate-source</option> declares all
+initialization functions as <type>extern</type>.  So they are exported
+unless this is prevented by a link script or other means.  Since libraries
+usually want to use the functions only internally it can be more useful to
+declare them as
+<link linkend="G-GNUC-INTERNAL:CAPS"><literal>G_GNUC_INTERNAL</literal></link>
+which is what <option>--internal</option> does.
+</para></listitem>
+</varlistentry>
+
 </variablelist>
 </refsect1>