Makefile: Accept "DESTDIR" instead of "prefix=", too

That way it adheres more commonly used conventions and allows using dh_auto_install
Revert "Install upstream ChangeLog only if it exists."

This reverts commit 35477c6172f62400120eb15f50b6819befd6f5f2.

If the upstream ChangeLog did not exist, the debian/changelog wasn't
installed either. Which was not the intention.
Remove explicit mentioning of upstream changelog in debian/rules

dh_installchangelogs finds it by itself if it exist. That way building
out of a freshly clone git repository does not fail due to a missing
upstream changelog.
Use dh_auto_{install,clean}; use "$(MAKE)" instead of "make"
Fix copy & paste error in binary-* targets in debian/rules
No more explicitly call "make manpages"

It's a depedency of "make install" in the next line anyway.