external: Add fallback logic from OpenConnect android/Makefile

I hoped this was not needed anymore, but the same bug has popped up again
on my build machine.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
external: Add "make sources" target

This lets F-Droid download and archive all prerequisities before starting
the build.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Update changelog and bump version to 0.80 (809)

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>