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.

## System-wide startup file for Octave.
##
## This file should contain any commands that should be executed each
## time Octave starts for every user at this site.
EXEC_PATH (cstrcat (fullfile (OCTAVE_HOME, 'notepad++'), pathsep, EXEC_PATH));
EXEC_PATH (cstrcat (fullfile (OCTAVE_HOME, 'bin'), pathsep, EXEC_PATH));
EDITOR (fullfile (OCTAVE_HOME, 'notepad++', 'notepad++.exe'));