Automate W32 installer building.
* doc/README.W32: Document new installer build procedure. * scripts/autogen.sh: Pass all args to the installer (regression fix). * scripts/conf-w32/README: Remove from repo. * scripts/conf-w32/bzip2-1.diff: Remove from repo. * scripts/mk-w32-dist: Rewrite. * scripts/w32installer.nsi [WITH_PATCHES]: Use constant patch file name.