Add build rule for python3 (bug #58689). * src/build-python3.mk: Add build rule for python3 version 3.8.3. * dist-files.mk: Add new file to list. * index.html: Add new package to list. * src/build-setuptools.mk: Update to version 44.1.1 (last version compatible with python2). Use python2. Set installation prefix. * src/build-mako.mk: Use python2. Set installation prefix. * src/build-markupsafe.mk: Use python2. Set installation prefix. * src/mesa.mk: Windows rule depends on scons. Use python2. * src/nsis.mk: Depend on scons. Use python2.

#!/usr/bin/env bash

prog=$1

if [ -z $prog ]; then
  echo "Missing program name" >&2
  exit 1
fi

cat << EOF
@echo off
set BINPATH=%~dp0
%BINPATH%bash -c '$prog %*'
EOF