maint - make 'rake deb' work again

It seemed to be that debian packages were built with the
ext/debian/patches/makefile.patch which creates a Makefile, but instead
debian/rules was calling on the Makefile in ext/

We can't really touch the Makefile in ext/ as that's also touched by the
openbsd port, but we can make a new one called Makefile.debian and call on that
instead.
Merge pull request #123 from richardc/bug/master/51

maint - make 'rake deb' work again

Fixes the issue @vincent-lauria notes in the comment to PR #51