Some useful references:

"Creating RPM Packages with Fedora"
   <https://fedoraproject.org/wiki/How_to_create_an_RPM_package>
"Packaging for openSUSE Leap"
   <https://en.opensuse.org/openSUSE:How_to_contribute_to_Leap>
"Build Service cross distribution howto"
   <https://en.opensuse.org/openSUSE:Build_Service_cross_distribution_howto>
"Fedora packaging guidelines"
   <https://fedoraproject.org/wiki/Packaging:Guidelines>

Alternative requirements:

As of 2017 there's some conflicting information on boolean operators
[1] says they are not supported in Requires
[2] says they are supported in all fields, including requires (rpm >= 4.13)
1: <https://fedoraproject.org/wiki/Packaging:Guidelines#Rich.2FBoolean_dependencies>
2: <http://rpm.org/user_doc/boolean_dependencies.html>
Fedora 25 has RPM 4.13
openSUSE Leap 42.2 has RPM 4.11