In MSVC builds, use 'rc' instead of 'windres'.

* INSTALL.windows (MSVC): Remove requirement to install mingw tools.
Instead, show how to set the PATH.
* lib/Makefile.in (libiconv.res.lo): On MSVC, use 'rc' and 'cvtres'.
* src/Makefile.in (iconv.res): Likewise.