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.