Changes for azn-linux

No systemd, more recent ruby, libselinux-ruby
Previously, RPMS built on Amazon Linux would
point to the improper ruby directory, or
would have improper dependencies.

Use %{amzn} as a rpm macro to determine if the
rpm is being built on an Amazon Linux distro.

If %{amzn} is set, use the Fedora/RHEL7 method
of discovering ruby lib directory. Use the
RHEL 6 init system (sysvinit). Use the RHEL 5
libselinux-ruby dependency.
Merge pull request #2621 from jeffb-bt/feature/azn-linux-build-3.5.1

Build RPMS properly on Amazon Linux