* 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