Changes to support MoltenVK.

[tomgreen66]

The tidied up changes suggested in #150 where extension updates required MoltenVK to be enabled as an allowed device using new extensions and flags - see: MoltenVK Runtime UserGuide

Had to fix Makefile also since VULKAN_SDK should include macOS directory e.g. from https://vulkan.lunarg.com/doc/sdk/1.3.224.1/mac/getting_started.html