* prj-part.adb, prj.adb, prj.ads, prj-nmsc.adb, prj-conf.adb
(Processing_Flags.Require_Obj_Dirs): new field, which controls whether
object directories must be present. In the case of gprclean at least,
these are optional (if they do not exist there is nothing to clean)
From-SVN: r150827
