RedHat has made it difficult to enforce which version of the OS your RPM should be installed on. This is my solution to these rather thorny problems.

snippet.spec

# Determine the OS version across the various forks using %{dist} detail from the building machine's kernel release.
# Versions 6 & 7 started including including the architecture in the kernel **release** field. We have to deal with that stupidity too.
%define osVersion %(uname -r | awk -F '.' '{for (i=NF; i > 0; i--) if ($i !~ /^(x86_64|i[36]86)$/) { print gensub(/^[^0-9]+/, "", "", $i); exit; }}')

# I wish redhat-release provided the major number, but that only started with 7 from what I've found. Even after adding a number to the redhat-release provide, they started using 7.0, just to make things difficult.

# Add conflicts as appropriate for the various RedHat major versions.
%if 0%{?osVersion} == 5
Conflicts: upstart, systemd
%endif
%if 0%{?osVersion} == 6
Conflicts: SysVinit < 2.86-24, systemd
# upstart provides SysVinit = 2.86-24, and we don't want to conflict with that here.
%endif
%if 0%{?osVersion} == 7
Conflicts: SysVinit, upstart
%endif

# 64 bit RPMs won't install on 32 bit systems already, the block below prevents the opposite too.
%ifnarch x86_64
Conflicts: /usr/lib64
%endif