The URL to the tarball needs the git tag, that might be 1.54-rc1 instead
of 1.53.90. Allow to define it as a separate variable in the spec file.
It can be set as `git_tag_version %{real_version}` when they are
identical.
It is not really needed here in the upstream spec file, as the "Source"
line is commented out, but add it as a reference for donwstream spec
files.
3cba4f2627