build, configure.ac: misc Windows installer translations build improvements
In configure.ac, add --enable-windows-installer option (off by default), which should be set to generate the necessary files for the installer translations during the build. Using this option is only supported when building from git, since the installer files are not included in source tarballs. Rename setup.isl.desktop.in to setup.isl.in, and instruct intltool to treat it as an .ini file explicitly. Delete generated message files during make clean.
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