These errors occur because, as of C_IP4, some of the components in the XilinxCoreLib library (specifically the vfft models) have references to functions in the UNISIM library.
Because of this dependency, you must compile the UNISIM library before you can compile the XilinxCoreLib library.
Please refer to (Xilinx Answer #2561) for details on how to compile the UNISIM library models.