Escape %distprefix in the spec
    
    The point is %distprefix is to allow things like the forge macros to add
    things to %dist without overriding its value. But currently %distprefix
    gets expanded (to nothing) in the fedora-release specfile, which
    prevents it from appearing as desired in the installed macros.dist file.
    
    Signed-off-by: Jason Tibbitts <tibbs@math.uh.edu>
    Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>