build: align minimum build dependencies with ndn-cxx

 * Recommend gcc >= 7.4.0
 * Require clang >= 4.0, or Xcode >= 9.0 on macOS
 * Silence an ABI-related diagnostic message from gcc on armv7
 * Make graphviz optional for building documentation
 * Sync sphinx configuration

Change-Id: Iff1920b0b5580d58db9aeef11298f11e22d9b3cb
diff --git a/docs/named_data_theme/layout.html b/docs/named_data_theme/layout.html
index 16ae50f..aa25686 100644
--- a/docs/named_data_theme/layout.html
+++ b/docs/named_data_theme/layout.html
@@ -12,7 +12,7 @@
         <div class="row">
              <div class="three columns">
                   <div id="logo">
-                        <a href="http://named-data.net" title="A Future Internet Architecture"><img src="http://named-data.net/wp-content/uploads/cropped-20130722_Logo2.png" alt="" /></a>
+                        <a href="https://named-data.net" title="A Future Internet Architecture"><img src="https://named-data.net/wp-content/uploads/cropped-20130722_Logo2.png" alt="" /></a>
                   </div><!--logo end-->
              </div>
 
@@ -38,12 +38,12 @@
           <h3>{{ _('Table Of Contents') }}</h3>
           {{ toctree(includehidden=True) }}
 
-          <h3>{{ _('Additional documenation') }}</h3>
+          <h3>{{ _('Developer documentation') }}</h3>
           <ul>
-            <li class="toctree-l1"><a class="reference external" href="http://redmine.named-data.net/projects/nfd/wiki">NFD Wiki</a></li>
             <li class="toctree-l1"><a class="reference internal" href="doxygen/annotated.html">API documentation (doxygen)</a></li>
           </ul>
           {%- endblock %}
+
           {%- block sidebarsearch %}
           <h3 style="margin-top: 1.5em;">{{ _('Search') }}</h3>
           <form class="search" action="{{ pathto('search') }}" method="get">
@@ -74,7 +74,7 @@
         <div class="row">
             <div class="twelve columns">
 
-                <div id="copyright">This research is partially supported by NSF (Award <a href="http://www.nsf.gov/awardsearch/showAward?AWD_ID=1040868" target="_blank>">CNS-1040868</a>)<br/><br/><a rel="license" href="http://creativecommons.org/licenses/by/3.0/deed.en_US" target="_blank">Creative Commons Attribution 3.0 Unported License</a> except where noted.</div>
+                <div id="copyright">This research is partially supported by NSF (Award <a href="https://www.nsf.gov/awardsearch/showAward?AWD_ID=1040868" target="_blank>">CNS-1040868</a>)<br/><br/><a rel="license" href="https://creativecommons.org/licenses/by/3.0/deed.en_US" target="_blank">Creative Commons Attribution 3.0 Unported License</a> except where noted.</div>
 
             </div>
         </div>