There are two different cmake things called ROOT_GENERATE_LIBRARY:
The Root one and the FairRoot one.

The former gets included by calling find_package2(...ROOT...), the latter get included explicitly. If a user includes ROOTMacros before they do the find_package2(...ROOT...), the user will get an error that ROOT_GENERATE_LIBRARY is called with the wrong number of arguments. (May Cthulhu help a user who misspells ROOTMacros as RootMacros and gets only the version from Root altogether.)

I think it would be a great idea if FairRoot acquired their own prefix for their cmake macros/functions instead of sharing them with ROOT. From my understanding, the there is not really a CMake prefix assignment authority -- one can simply grab whatever prefix one likes. Using FAIRROOT_ as a prefix will make collisions with other projects less likely.