Currently gem2rpm generates multiple lines if one dependency required which leads to problems.
RPM 4.14 (since F27) will get ability to specify things like BuildRequires: (rubygem(foo) >= 5.1 with rubygem(foo) < 5.3), or even more complicated cases would work as well: BuildRequires: ((rubygem(foo) >= 5.1 with rubygem(foo) < 5.3) or (rubygem(foo_ext) >= 5.1 with rubygem(foo_ext) < 5.3))