Patch to de-copy-paste the makefiles

Hi

The attached patch generates all the makefile targets from a shared include
file.

It also allows WORKDIR= to be defined to change where the output is
produced.

This makes it easy to replace the actual build commands with something
else, in my case docker-rpm-builder. I haven't yet made that neatly
configurable/pluggable and I'm not sure I should; it's actually probably
simpler to use something completely
separate and just pass the same make macros.

So I probably won't do anything more with this.

Anyway, in case it's useful/interesting, here it is. Apply with "git am".