Only require cmake-rpm-macros when rpm-build is installed
    
    There are other use-cases for cmake besides building rpms, for which a hard
    requirement of rpm is absurd. Ironically enough, this hard dependency also
    causes an ugly conflict requiring --nodeps hacks when building rpm's own
    test-suite.
    
    A conditional dependency on rpm-build sorts the issue nicely with no
    regressions to the use-case of building packages.