After do a little more research on the topic, I just found out the following:
You can't set the compiler settings for FlexRIO, which is why they don't show up in the build options as they would for other FPGA targets. The reason for this is that FlexRIO FPGA code is usually run at a much higher speed and needs specific compiling options to make sure it operates correctly. Thus, we have fixed compiler options for FlexRIO.
Kareem W. National Instruments Web Product Manager