build: always build compilation DB option

[ken-matsui]

Due to performance concerns, building compilation DB is turned off by default. But depending on a project, we want to use up-to-date compilation DB always. It's tiring to do both poac b and poac b --compdb. So, it would be great if we could have an option in poac.toml (or maybe CLI flag? need discussion) to enable compilation DB build always.