Add support for FreeBSD

Since FreeBSD support was seemingly dropped for RingRacers because of what I can only guess is due to dropping Makefile support, I'm readding it again. Note that due to C++ shenanigans, the project must be compiled with -DENABLE_PRECOMPILED_HEADERS=OFF -DCMAKE_NO_SYSTEM_FROM_IMPORTED=ON passed to CMake in order to build, otherwise GCC/CLang won't be able to locate unistd.h.