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.
> I didn't like the idea of updating the Makefile for each minor release

The idea is that you run it like this: `make rpm rpm_release=2` to override the value in the Makefile -- you wouldn't actually edit the makefile.