Fix compilation of coq and plugins using coq_makefile under cygwin

Problems:
- strip may not be "strip" but "i686-bla-strip", hence we ask ocamlc -config
  the value of "ranlinb" and replace ranlib by strip obtaining
  "i686-bla-strip" from "i686-bla-ranlib"
- coq_makefile was not quoting the plugins/ paths
- coq_makefile was quoting twice camlpX (the shell of cygwin was confused)