build: skip $(IFLEXDIR) in mk_shell_and_ocamlbuild_config.sh.
    
    (Patch by Adrien Nader!)
    
    On Windows, IFLEXDIR is defined as -I"$(FLEXDIR)". The new
    mk_shell_and_ocamlbuild_config script outputs:
      echo IFLEXDIR="\"$(IFLEXDIR)\""
    
    However, becauses $(IFLEXDIR) contains quotation marks, this becomes:
      echo IFLEXDIR="\"-I"$(FLEXDIR)"\""
    
    This unquotes $(FLEXDIR) and breaks the program.
    
    Simply exclude IFLEXDIR from the variables that are handled by the
    script. This is actually what the previous scripts were doing: the
    exclude list was .*FLEXDIR and matched both IFLEXDIR and FLEXDIR while
    the one I made only had \<FLEXDIR\> in it.
    
    I've tried to handle it but there are at least three competing languages
    and quoting rules: makefile (both gnu make and several bsd makes), shell
    script and ocaml.
    There's no human way to do string processing given the portability
    constraints: both gnu make and pmake (and its descendants) have powerful
    string processing functions but they're not the same.
    The only sane way would be to store the configuration in a more evolved
    language that is portable, has arrays and powerful string handling
    routines in standard like C or awk. From there it would be possible to
    output make, shell script and OCaml code easily.
    
    One day. Maybe.
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14171 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02