[gtkmm-documentation] Headers and Linking section: Replace broken link

[Kjell Ahlstedt <kjellahl src gnome org>]

commit c0bce5c7f10f1606b0365abce26d2c3139051d52
Author: Kjell Ahlstedt <kjell ahlstedt bredband net>
Date:   Wed Feb 1 18:31:21 2017 +0100

    Headers and Linking section: Replace broken link
    
    Replace the link to Openismus with links to GNU's documentation of
    autoconf and automake. Bug 777879

 docs/tutorial/C/index-in.docbook |   10 +++++++---
 1 files changed, 7 insertions(+), 3 deletions(-)
---
diff --git a/docs/tutorial/C/index-in.docbook b/docs/tutorial/C/index-in.docbook
index d4d955a..39f8b76 100644
--- a/docs/tutorial/C/index-in.docbook
+++ b/docs/tutorial/C/index-in.docbook
@@ -395,11 +395,15 @@ Note that the API name does not change for every version because that would be a
 API and ABI break. There might be a future gtkmm-5.0 API which would install in parallel
 with gtkmm-4.0 without affecting existing applications.
 </para>
-
 <para>Note that if you mention extra modules in addition to gtkmm-4.0, they should be separated by spaces, 
not commas.
 </para>
-<para>
-Openismus has more <ulink url="http://www.openismus.com/documents/linux/automake/automake.shtml";>basic help 
with automake and autoconf</ulink>.
+
+<para>The GNU site has more information about <ulink 
url="https://www.gnu.org/software/autoconf/";>autoconf</ulink>
+and <ulink url="https://www.gnu.org/software/automake/";>automake</ulink>.
+</para>
+<para>If you start by experimenting with a small application that you plan to use just for yourself,
+it's easier to start with a Makefile similar to the <filename>Makefile.example</filename> files
+in the <link linkend="chapter-building-applications">Building applications</link> chapter.
 </para>
 
 </sect1>