A parameter called "XOR Manipulation" will set the XOR usage for the compiler. This is available in the implementation options. (Right-click on Implement Design, select Properties | Optimization Tab | XOR Synthesis.)
A pull-down menu allows you to change the parameter, the valid options are: All, Explicit, or None. When this parameter is set to All, all XOR functions found in the source file will be implemented using the hardware XOR. When this parameter is set to None, the hardware XOR is never used. Setting this parameter to Explicit essentially does the same thing as if you set it to All.
XPLA Professional
A parameter called "XOR Manipulation" allows you to set the XOR usage for the compiler. To set this parameter, press the "Properties" button in the main GUI of XPLA Professional.
The next window will have a pull-down menu that allows you to change the parameter, the valid options are: All, Explicit, or None. When the parameter is set to All, all XOR functions found in the source file will be implemented using the hardware XOR. When this parameter is set to None, the hardware XOR is never used. Setting this parameter to Explicit essentially does the same thing as if you set it to All.