Fix undefined variables in coq_makefile
Fix undefined variables in coq_makefile

Detected by running plugin_tutorial from the main makefile which has
--warn-undefined-variables on.

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