* prj-conf.adb (Parse_Project_And_Apply_Config): Reset incomplete
with flags before parsing the projects.
* prj-err.adb (Error_Msg): Do nothing if there are incomplete withs.
* prj-part.adb (Post_Parse_Context_Clause): Set Incomplete_Withs
to True in the flags, when Ignore_Missing_With is True and an
imported project cannot be found.
* prj-proc.adb (Expression): When there are incomplete withs and
a variable or attribute is not found, set the variable/attribute
to unknown.
* prj.ads (Processing_Flags): New flag Incomplete_Withs,
defaulted to False.