Vulkan: Changes to CMake config.

Paths to vulkan libraries, paths and related components were
hardcoded in the platform cmake file. This patch separates
this by using adding CMake modules for Vulkan and ShaderC.

This change has only been applied to the macOs configuration as
that is currently our main platform for development. Other platforms
will be added during the development of the Vulkan back-end.

extern/vulkan_memory_allocator/CMakeLists.txt

@@ -7,6 +7,7 @@ set(INC
 
 set(INC_SYS
   ${VULKAN_INCLUDE_DIRS}
+  ${MOLTENVK_INCLUDE_DIRS}
 )
 
 set(SRC