Revert "Remove incomprehensible Debian magic"

This reverts commit ba60c400ae5c69bd8b7f6c1368e6e8c6bfb42f24.
Bring back a waf-compatible deiban build system

For override_dh_auto_install, just tell waf to install the files under debian/mpv.

The main problem was that the script names clean,build,install in the
root directory collided with makefile targets in debian/rules which lead
make to do nothing since they appeared already up-to-date. .PHONY wasn't
enough since the targets are implicit via the % operator. I had to
explicitly declare the problematic targets AND list them as .PHONY