A somehow working makefile

This is again based on @NickStephens's work in https://github.com/NickStephens/sensibility-jni/

Alas I don't know if my thing builds anything useful. (I still managed
to fix one small issue that existed in the original build file and
caused complaints from the `ndk-build` script. We should to use
`LOCAL_SHARED_LIBRARIES` to add the Python `.so`, not `LOCAL_LDLIBS`
which should be used exclusively for system libraries).