Fix BZ#5785 (make install -j broken)
Fix BZ#5785 (make install -j broken)

This adds back `$(MKDIR) $(FULLCOQLIB)/toploop`, which was lost between
8.6 and 8.7.

Zimmi48/coq,ppedrot/coq,maximedenes/coq,coq/coq,SkySkimmer/coq,JasonGross/coq,JasonGross/coq,letouzey/coq-wip,Matafou/coq,maximedenes/coq,gares/coq,ppedrot/coq,ejgallego/coq,letouzey/coq-wip,gares/coq,ppedrot/coq,ejgallego/coq,SkySkimmer/coq,gares/coq,Zimmi48/coq,silene/coq,Zimmi48/coq,silene/coq,coq/coq,coq/coq,JasonGross/coq,maximedenes/coq,ejgallego/coq,silene/coq,gares/coq,Matafou/coq,maximedenes/coq,SkySkimmer/coq,coq/coq,maximedenes/coq,letouzey/coq-wip,Matafou/coq,ppedrot/coq,SkySkimmer/coq,Zimmi48/coq,gares/coq,ejgallego/coq,letouzey/coq-wip,ppedrot/coq,JasonGross/coq,letouzey/coq-wip