[CentOS] Single source repo, producing packages for CentOS 6 and 7, how?

Guys,

 I have a source code here, that produces RPM packages for CentOS 6, that
includes an Upstart Job and a ugly and minimal Init Script.

 Now, I have it running on CentOS 7, via systemd, looks good!

 However, my RPM package for CentOS 7 still includes the useless Upstart
and Init Scripts...

 So, here is the question:

 How are you guys managing this? I mean, I would like to have 1 single
source and 1 SPEC file for both CentOS 6 and 7, but no need to systemd
service files on 6 / no need to upstart job files on 7.

Cheers!
Thiago