You will need to create separate packages. This is because of the potential of different library versions being required in various versions of Ubuntu.
As well, when you build the packages into the binaries, it will be building only for the libraries available within that version of Ubuntu.
Subsequently, this is why in debian/changelog you are only allowed to specify one version of Ubuntu at a time on the version line (for example: rkhunter (1.3.8-7~natty1~ppa1) natty; urgency=low - note the natty part changes to maverick, lucid, oneiric, hardy, etc.)
Take a look at the files in my rkhunter backports PPA: https://launchpad.net/~trekcaptainusa-tw/+archive/rkhunter Feel free to see what the changelog entries show, and how I named the package version numbers (you can omit the ~ppa# part at the end, but you should append the version of ubuntu to the package names if you're building for more than one version of Ubuntu, and then build each separately).
EDIT
To better see what I mean, I urge you to compare the actual debian/control files, and the debian/changelog files for the version of rkhunter in my PPA (for Lucid), and compare that to the source version in Oneiric's repos right now, you'll notice the change(s) that had to be done in order to get it to build in Lucid (such changes are common, so when building for Natty, for instance, you'll be specifying newer package versions than in Lucid or Maverick or others. The same is said for building for Oneiric, and backporting to Natty, you sometimes have to specify different versions of build dependencies, among other things, for the older version(s) of Ubuntu)
