1. `make tools` will not updated `clog` when `clog/main.go` was changed,
except `${GOBIN}/clog` or `tools-stamp` was deleted. `gosum` has the
same problem.
2. $(GOBIN)/gosum was just like magic number.
Amend targets naming convention as per [1]:
- Switch from underscore '_' separated naming scheme to dash '-'
separated naming scheme.
- Replace test with check.
Simulations now live in their own Make module.
[1] https://www.gnu.org/prep/standards/html_node/Standard-Targets.html
