External libraries for Windows are now built automatically in the
qpdf/external-libs repository and include openssl in addition to zlib
and jpeg. Use these, and update the Windows build to build with the
openssl crypto provider by default. We leave the native crypto
provider enabled in case there is a problem with openssl and also to
continue to exercise that code.
Update the build to support new external-libs layout, and autoconf
options to specify windows word size. Split make_windows_releases
into multiple scripts.
