Use fedora/rhel7's ruby dir on amz-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.
Merge pull request #190 from jeffb-bt/feature/azn-linux-build-1.3.2

Build RPMS properly on Amazon Linux