The Sphinx-3.0 c:function:: directive requires the return type. Sphinx-3.0 also adds function-like macro support to the c:macro:: directive, which Sphinx-1.x and Sphinx-2.x do not support, but it is probably a good idea to keep compatibility with slightly older Sphinx for now.
To build the documentation, invoke
make html
Then point your browser to _build/html/index.html.