I just installed MacOS Sierra 10.12 Beta 3 on a Mac Mini with Xcode 8 Beta 4 and thought I would give OF a shot.
Unfortunately as of 10.12, the Quicktime Framework has been removed from Xcode. I suspect this will be the case if you try to build any old OF version in 10.12 when all the stuff moves forward.
The quick fix options would be to get the 10.11 SDK package and put that in Xcode to build against Quicktime. The alternative is going to be removing all quicktime code or put in some compiler flag to ignore those files in 10.12. Things with Quicktime will still run in 10.12 (for now)
To get the SDK Package for 10.11 on a fresh macOS and Xcode install, you can get it from a previous version by doing:
Xcode.app-> Right Click "Show package Contents" -> Contents -> Developer -> Platforms -> MacOSX.platform -> Developer -> SDKS -> MaxOSX10.11.sdk
Once you have that in there (and have closed and opened Xcode) - you will need to change your project settings as well - In Project settings -> Build Settings (make sure you have selected "All" at the top) -> Base SDK-> OS X 10.11 (not "Latest")
You will also need to change the openFrameworks Project file (in your project in the openFrameworks folder - select the openFrameworksLib.xcodeproj) and go to build settings and set that Base SDK to OS X 10.11 as well