Adding FreeBSD build support.

[LadySerenaKitty]

Making all changes required to build on FreeBSD. Unity is not available for FreeBSD, so all Unity tests are disabled for FreeBSD. Some CMake entries for Linux may have changed, but their functionality remains unchanged.

[LadySerenaKitty]

Requested changes made, should be ready for merge.

[LadySerenaKitty]

Everything builds up to this point:

[ 91%] Building CXX object DiligentSamples/Samples/GLTFViewer/CMakeFiles/GLTFViewer.dir/src/GLTFViewer.cpp.o
/usr/home/jlhawkwell/Projects/DiligentEngine/DiligentSamples/Samples/GLTFViewer/src/GLTFViewer.cpp:1218:23: error: no member named 'ComputeMotionVectors' in 'Diligent::EnvMapRenderer::RenderAttribs'
        EnvMapAttribs.ComputeMotionVectors = m_bEnablePostProcessing;
        ~~~~~~~~~~~~~ ^
1 error generated.
gmake[2]: *** [DiligentSamples/Samples/GLTFViewer/CMakeFiles/GLTFViewer.dir/build.make:76: DiligentSamples/Samples/GLTFViewer/CMakeFiles/GLTFViewer.dir/src/GLTFViewer.cpp.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:5244: DiligentSamples/Samples/GLTFViewer/CMakeFiles/GLTFViewer.dir/all] Error 2
gmake: *** [Makefile:156: all] Error 2

[TheMostDiligent]

Everything builds up to this point:

Please update all 4 submodules to the latest versions.

[LadySerenaKitty]

Everything builds up to this point:

Please update all 4 submodules to the latest versions.

I did that, re-applied some of my changes, and then this happened:

[ 91%] Linking CXX executable Atmosphere
ld: error: unable to find library -lX11
ld: error: unable to find library -lxcb
c++: error: linker command failed with exit code 1 (use -v to see invocation)
gmake[2]: *** [DiligentSamples/Samples/Atmosphere/CMakeFiles/Atmosphere.dir/build.make:178: DiligentSamples/Samples/Atmosphere/Atmosphere] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:5181: DiligentSamples/Samples/Atmosphere/CMakeFiles/Atmosphere.dir/all] Error 2
gmake: *** [Makefile:156: all] Error 2