Before this change, release.sh built some native stuff, and
Makefile.CrossWindows built the Windows releases.
Instead, use a single release.make to build the whole release. It also
avoids to build the server one more time.
Add a script to generate the whole release properly.
It first builds locally in release mode, then execute tests. Then it
builds archives for Windows. Finally, it puts all release files (Windows
archives, prebuilt server and checksums) in a separate release
directory.