Use ruby lib from fedora and new rhel for amazon linux
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
rubygem-rdoc package dependency instead of
ruby-rdoc
Merge pull request #675 from jeffb-bt/feature/azn-linux-build-2.0.1

Build RPMS properly on Amazon Linux