This looks for VULKAN_SDK at build time, and if present it tells
Filament to use the `libvulkan.1.dylib` that's located there. Otherwise
it falls back to our old loading strategy, which uses a bundled version
of MoltenVK that allows us to avoid requiring installation of the
LunarG SDK.
I tried this out an it works for me...
Closes issue #238.
