doc: build: generate revision.sty each time a PDF document is compiled.
doc: build: generate revision.sty each time a PDF document is compiled.

The previous scheme was lazy on purpose: avoid regenerating the output
each time the revision is updated.  But it was too lazy, and not updated
frequently enough.  This scheme seems the best one: we always have a
correct revision information when we compile the documentation.

	* doc/Makefile.am: here.

aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi