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.
Merge pull request #6 from psi/amazon-linux

Fix build.sh to work on Amazon Linux.