tsqlquery.applyupdate refer to unit BufDataset of package FCL.
The name FCL is used with 2 meanings. The "package FCL" (Lazarus package) only contains registration stuff. But not the unit you referred to.
The actual FCL, is in the FPC installation. This can not be rebuild from inside the IDE.
You need to rebuild FPC for this.
Sometimes you can get away by moving units from one fpc folder, and recompile those only. But that might be tricky...
Best is to to a 2nd fpc install from source, using a tool like fpcup