bin/compile: Automatically update Release field in spec file

Look at git tags to determine if this is a release, post-release, or
pre-release build, e.g.,

    release       1{%?dist}
    post-release  1.20131124015959.dev.g81c9ae9%{?dist}
    pre-release   0.20131124015959.dev.g81c9ae9%{?dist}