That’s cause you modified some files, you can see what the changes are on the output window. If you hadn’t done any change on purpose, you may have touched some file or something, if it detect even the slightest change it will complain.
Anyway, I messed up, should have created a new branch while I worked on 4.8 and then re-merged. Until I get 4.8 working, please download the code from here https://.com//UnrealEngine/tree/a6e78633a2e54b8cd0895b7202a0253260464fd0 as a zip. The zip is about 150 . Just copy the engine folder from the zip to the folder, replace everything, and then recompile.