build+docs: align minimum requirements with ndn-cxx/NFD

 * Recommend boost >= 1.65.1 and gcc >= 7.4.0
 * Require clang >= 4.0, or Xcode >= 9.0 on macOS
 * Silence an ABI-related diagnostic message from gcc on armv7
 * Update waf to version 2.0.21
 * Make graphviz optional for building documentation
 * Sync sphinx configuration

Change-Id: Ib271da841cd12e7595ae83d75bd0fc681f8c231f
diff --git a/docs/index.rst b/docs/index.rst
index aa2102d..6e2a4b4 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -3,7 +3,7 @@
 
 NLSR is a routing protocol in NDN that populates NDN's Routing Information Base. NLSR will
 continue to evolve alongside the Named Data Networking `protocol
-<https://named-data.net/doc/ndn-tlv/>`_.
+<https://named-data.net/doc/NDN-packet-spec/current/>`_.
 
 NLSR is an open and free software package licensed under the GPL 3.0 license and free to
 all Internet users and developers. For more information about the licensing details and
@@ -23,11 +23,11 @@
 the `NLSR Paper`_.
 
 NLSR Documentation
--------------------
+------------------
 
 .. toctree::
    :hidden:
-   :maxdepth: 3
+   :maxdepth: 2
 
    GETTING-STARTED
    INSTALL