Small configure cleanup: prefs not mutable after args parsing
Small configure cleanup: prefs not mutable after args parsing

+ small cleanup of prefix manipulation around one_instdir (constant behaviour)

Zimmi48/coq,coq/coq,coq/coq,SkySkimmer/coq,Zimmi48/coq,SkySkimmer/coq,SkySkimmer/coq,ejgallego/coq,SkySkimmer/coq,ejgallego/coq,Zimmi48/coq,coq/coq,coq/coq,ejgallego/coq,Zimmi48/coq,ejgallego/coq