Makefile: [fix] allow minor version releases by using wildcard expansion

Previously, incrementing the Release number in stb-tester.spec.in to
anything other than 1 would cause the srpm make target to break because
the name of the produced RPM would not match that which was hardcoded to
the "src_rpm" variable.

This commit fixes the issue by reducing the "src_rpm" variable to just the
suffix (which is constant) and using make target expansion and wildcards.

Fixes #241.
On my Ubuntu system this causes `make` to print `make: rpm: Command not found` (even when I run a target like `make check` that has nothing to do with rpm). I think we want a variable hard-coded to `fc20`, but that you can override on the command-line with `make rpm fedora_release=fc20`, similar to how we do things for the ubuntu packages.