Make --enable-werror work properly on msvc, handle extra warnings
flags for msvc in configure.ac instead of hardcoding into
make/msvc.mk, separate warnings flags into WFLAGS in autoconf.mk to
avoid duplication and to make it easier to override.
Update the build to support new external-libs layout, and autoconf
options to specify windows word size. Split make_windows_releases
into multiple scripts.
