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/conf.py b/docs/conf.py
index 8dbae78..76260ff 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -1,10 +1,8 @@
-# -*- coding: utf-8 -*-
-#
 # Configuration file for the Sphinx documentation builder.
 #
 # This file only contains a selection of the most common options. For a full
 # list see the documentation:
-# http://www.sphinx-doc.org/en/master/config
+# https://www.sphinx-doc.org/en/master/usage/configuration.html
 
 # -- Path setup --------------------------------------------------------------
 
@@ -20,13 +18,13 @@
 # -- Project information -----------------------------------------------------
 
 project = u'NDNS: Domain Name Service for Named Data Networking'
-copyright = u'Copyright © 2014-2020 Named Data Networking Project.'
+copyright = u'Copyright © 2014-2021 Named Data Networking Project.'
 author = u'Named Data Networking Project'
 
-# The short X.Y version
+# The short X.Y version.
 #version = ''
 
-# The full version, including alpha/beta/rc tags
+# The full version, including alpha/beta/rc tags.
 #release = ''
 
 # There are two options for replacing |today|: either, you set today to some
@@ -40,7 +38,7 @@
 
 # If your documentation needs a minimal Sphinx version, state it here.
 #
-needs_sphinx = '1.1'
+needs_sphinx = '1.3'
 
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
@@ -87,6 +85,12 @@
 # so a file named "default.css" will overwrite the builtin "default.css".
 html_static_path = ['_static']
 
+html_copy_source = False
+html_show_sourcelink = False
+
+# Disable syntax highlighting of code blocks by default.
+highlight_language = 'none'
+
 
 # -- Options for LaTeX output ------------------------------------------------