Obsolete building in source dir?

[Giovanni Bajo]

Hello,

Do we still need to support builds in source directory? It looks like it does
not work anymore not even on primary targets because of recent changes. We
discouraged doing it for a long time already, and no developers probably test
this so it is going to be often (always) broken. Maybe is it time to obsolete
(or forbid) this feature for 4.0.0, by having configure emit an error and exit?

Giovanni Bajo