Making NO_CLEAN configurable through src.conf, possibly default

[Eugene Grosbein]

17.12.2019 16:54, Ed Maste wrote:

> For several years I've built with NO_CLEAN, and would like us to
> consider making it the default in the future. It used to be we
> regularly made changes in the tree that required a clean build
> (because of missing explicit dependencies, or because of limitations
> in the way make automatically tracks dependencies), but for the last
> couple of years we've added explicit workarounds when that happens.
> 
> As a first step I've proposed having it be set via src.conf WITH_CLEAN
> / WITHOUT_CLEAN knobs, in review https://reviews.freebsd.org/D22762.
> What do folks think (of WITH_/WITHOUT_CLEAN and of making it the default)?

Major upgrades or switching to META_MODE from older style still need clean obj directory.