make: enable `assert()` in `make scan-build`

Compare scan-build's [documentation][1]:

> ALWAYS analyze a project in its "debug" configuration
>
> Most projects can be built in a "debug" mode that enables assertions.
> Assertions are picked up by the static analyzer to prune infeasible
> paths, which in some cases can greatly reduce the number of false
> positives (bogus error reports) emitted by the tool.

Closes #3960.

  1: http://clang-analyzer.llvm.org/scan-build.html#recommended_debug