manual: fix parallel make in manual/library
manual: fix parallel make in manual/library

The rule said:

$(INTF): $(MLIS)
	$(OCAMLDOC) -latex [...] $(MLIS)
        # renaming of files
       	mv -f Arith_status.tex Arithstatus.tex
	mv -f Big_int.tex Bigint.tex
	mv -f Ast_helper.tex Asthelper.tex
	mv -f Ast_mapper.tex Astmapper.tex

If two targets executed at the same time depend on different files in
INTF, this rule would be executed several time in parallel, and in
particular the calls to `mv -f ...` would race with each other,
resulting in a build failure.

Now the $(OCAMLDOC) call is made unique by using a single target:

$(INTF): interfaces
interfaces: $(MLIS)
	$(OCAMLDOC) -latex [...] $(MLIS)
        [...]

Note that parallel build does not improve the build speed on my
machine; but it can still make people more productive by not breaking.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml