optional require [Graphviz](https://stackoverflow.com/...)’s dot to gen dep graph

WARN_IF_INCOMPLETE_DOC=NO

HIDE_IN_BODY_DOCS=YES

NOTE: EXTRACT_ALL shall be what you want (otherwise one with no doc will not shown (as C has no primitive of export)

but it’ll also gen doc for funcs in C

NOTE: if a enum has no doc on top, undocumented enum vals won’t show up

I’ve installed in 58deb, tencent-ub