Fix build.sh to work on Amazon Linux.

On Amazon Linux, `buildsys-macros` providing `/etc/rpm/macros.dist` does
not exist. Rather, `system-release` provides `/etc/rpm/macros.disttag`.
Apart from failing the check for `/etc/rpm/macros.dist` in build.sh, the
RPM builds fine. This patch changes build.sh to look for either file.

I ran into this while using [Opscode's runit cookbook][0], which now bundles runit-rpm and uses it to build runit on RHEL-family platforms.

[0]: https://github.com/opscode-cookbooks/runit