Avoid concurrent runs when producing html documentation (bug #5269).
Avoid concurrent runs when producing html documentation (bug #5269).

Make does not allow for rules that produce multiple outputs (unless they
are pattern rules). As a consequence, the hacha rule could be run several
times concurrently, thus causing doc/refman/html to be deleted under the
feet of some runs.

This commit fixes the issue by turning the rule into a single-output rule
and adding a dummy rule to handle all the other indexes. Note that this is
not a perfect solution since, if the user were to manually delete one of
the auxiliary index, it would not be rebuilt until the main index is.

SkySkimmer/coq,coq/coq,ejgallego/coq,coq/coq,ppedrot/coq,ppedrot/coq,maximedenes/coq,silene/coq,gares/coq,maximedenes/coq,william-lawvere/coq,JasonGross/coq,ppedrot/coq,gares/coq,Matafou/coq,william-lawvere/coq,Lionel-Rieg/coq,gares/coq,Ptival/coq,Zimmi48/coq,amintimany/coq,JasonGross/coq,gares/coq,silene/coq,amintimany/coq,Matafou/coq,JasonGross/coq,SkySkimmer/coq,letouzey/coq-wip,Lionel-Rieg/coq,SkySkimmer/coq,letouzey/coq-wip,Zimmi48/coq,ppedrot/coq,Zimmi48/coq,ejgallego/coq,gares/coq,Ptival/coq,amintimany/coq,SkySkimmer/coq,Ptival/coq,JasonGross/coq,ppedrot/coq,coq/coq,maximedenes/coq,letouzey/coq-wip,ejgallego/coq,ejgallego/coq,Lionel-Rieg/coq,Zimmi48/coq,coq/coq,silene/coq,maximedenes/coq,william-lawvere/coq,maximedenes/coq,letouzey/coq-wip,Matafou/coq,letouzey/coq-wip