build: replace ocamlcomp*.sh.
    
    This script was built from ocamlcomp.sh.in through sed and is called
    instead of "ocamlc" (for instance).
    It makes it possible to switch from "ocamlc" to "ocamlc.opt" without
    changing anything in the Makefiles, only calling sed.
    
    I couldn't cleanly make it handle both a compiler for the target and for
    the build. Instead I'm replacing it and doing as much as possible
    directly in the Makefiles.
    I hoped it would reduce the number of shell invocations, which would
    speed things up quite a lot on Windows but I still had to have at least
    one since it's not possible to update a make variable from inside a make
    rule: i.e. it's not possible to do X=a, build a.opt and update X to be
    a.opt.
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14168 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02