I put almost 3 hours on this!
It ships with a lot of makefiles for various platforms, notably
one for android_arm. I patched it a bit because Termux already gives
many of the goodies. Probably it could be simplified.
Most of the time went on tinkering with flags and waiting for the
Docker container to reload the new files...there must be a better way!
p7zip doesn't seem to honor $prefix, instead it uses $DEST_HOME.