rpmbuild: provide the "dynamic" %buildtag
    
    The %buildtag macro can be used similarly to %dist, as post-release
    flag.  While the %dist tag is specified by the mock build-chroot,
    %buildtag is specified by the wrapping build system.
    Example of use is:
    
        Release: 0%{?dist}%{?buildtag}
    
    This could solve the "release autobump" problem.  The use of this macro
    is not at the time of committing this mandatory, nor suggested.  This is
    mostly for R&D to see how it serves its purpose.
    
    This commit is reaction on discussion on Fedora devel:
    https://lists.fedoraproject.org/archives/list/\
    devel@lists.fedoraproject.org/thread/TXLF22CSUVUIQBVHH2NEFF4IOIFHS5WK/
    
    When the %buildtag is in use, simple re-build (resubmit button in
    copr UI) of the package causes that the Release is always bumped.
    
    Except that dnf/yum should newly always prefer the latest build of the
    affected package in such situation, it also guarantees that our build
    garbage collector (prunerepo) really keeps only the latest available
    builds of each package (so far, when there are two exactly the same
    NVRs, it is not defined which one of those is to be deleted).
    
    Fixes: #840