one has the date in the second line while the other has not
This is normal, to build a release that do not include the date, you would have to set the option -DSlicer_RELEASE_TYPE:STRING=Stable. See ReleaseProcess checklist.
But the impact is only cosmetic.
If anyone was able to build a compatible Slicer
You can not build extension using Ubuntu 16.04 and expect them to be compatible with the Slicer 4.8.1 release.
Indeed, the Slicer 4.8.1 release was done using Ubuntu 10.04.
Considering it is not possible to install such an old distribution, there is we now docker images allowing to reproduce builds.
Following this guide will allow you to build your extension and have it working with the Slicer 4.8.1 Linux release.