fix RPM conditional on RHEL3 and RHEL4

For some reason older RPM doesn't think the conditional is valid syntax:

    error: /builddir/build/SPECS/beah.spec:34: parseExpressionBoolean returns -1

I noticed that it can just use the %{with_systemd} variable anyway.

Change-Id: If4bbd098e57ced55db4c248e74afa0a67be22d16