Why does Simulink default to LCC Compiler instead of using Xilinx SDK toolchain and how do I correct the tool chain?

I have a wrapper function that contains C Code. I would like to deploy it on a USRP E310. Support packages are all properly installed. However, when I try to compile a function that has a POSIX feature (ie: clock_gettime()), the lcc_win64 compiler is invoked. That ultimately fails as clock_gettime() is a UNIX function.

After some more research, I noticed that the Xilinx SDK tool chain doesn't validate successfully when performing a tool chain Validation. NOTE: The LCC compiler validates perfectly. Xilinx reports "undefined macros for: $(XILINX_ARM_CROSS_COMPILE)'... among others.

My two questions are: how do I get the Xilinx tool chain to properly register? Why would the LCC compiler invoke first even when in my configuration set, I'm specifically calling out to use Xilinx SDK Tools?