configure: add support of MinGW Win32 environment (fix #2526)

 * Since MinGW/Msys doesn't provide a cygpath utility, we emulate
   it via an ocaml script tools/mingwpath.ml
 * Avoid the crazy sed portions for backslash escaping, instead use
   a more robust ocaml script tools/escape_string.ml based on
   String.escaped
 * No more config/Makefile.template + sed on it, but rather a
   "cat << EOF > ..." as for config/coq_config.ml
 * Normally, support of Cygwin should be preserved, as well as
   mingw32 cross-compilation from linux (cf. myocamlbuild.ml)

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@15348 85f007b7-540e-0410-9357-904b9bb8a0f7