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.
This won't build the src rpm file (if it doesn't already exist) when you run `make rpm` -- unless I'm missing something. When testing this make sure you remove any existing source rpm files lying around to see if `make rpm` can still generate everything from scratch.

Note that the `%` doesn't work exactly like a shell wildcard -- according to [this](https://www.gnu.org/software/make/manual/html_node/Pattern-Intro.html) there must be a `%` in the target, not just in the pre-requisites. See also [Static Pattern Rules](https://www.gnu.org/software/make/manual/html_node/Static-Pattern.html) and [Substitution References](https://www.gnu.org/software/make/manual/html_node/Substitution-Refs.html) for other similar, but different, meanings of `%`.

Instead, I recommend adding a variable `rpm_release` or something like that, setting `src_rpm=stb-tester-$(ESCAPED_VERSION)-$(rpm_release).fc20.src.rpm`, and setting this version in `stb-tester.spec` (note that `stb-tester.spec` is already generated from `stb-tester.spec.in` by [this rule](https://github.com/drothlis/stb-tester/blob/82a3448/Makefile#L58) so it'll be easy to add one more substitution).

While you're at it, it would be great if you make it so that the Fedora version (20) isn't hardcoded.