Adjusted build system to correctly build DLLs for windows.

Adjusted build system to correctly build DLLs for windows.

Adjustments:
    - configure.ac:  pass win32-dll option to LT_INIT

    - Makefile_libre2c_posix.am: use -no-undefined in LDFLAGS

    - use slibtool: https://github.com/midipix-project/slibtool for windows
      builds The problem with libtool is that it doesn't allow to link libstdc++
      and libgcc statically, which is necessary to build portable DLLs with
      Mingw. Libtool adds -nostdlib option to LDFLAGS and links some predefined
      objects that pull in dependency on dynamic libstdc++ and libgcc, even in
      the presence of -static-libstdc++ -static-libgcc.