This attempts to fix builds against GTK builds that have Vulkan enabled, since GTK's headers will pull in the Vulkan SDK headers as a result. This adds an optional dependency that will pull in the include path for Vulkan SDK's headers, so that things will build if GTK is built with Vulkan. This follows the way that GTK uses to find the Vulkan SDK.
For the NMake Makefiles, it already has a simple mechanism that checks for the
VULKAN_SDK envvar that is set by the Lunar G Vulkan SDK installer, or can be overridden so that the Vulkan SDK headers could be found. I did not update the autotools items for this one since I am not sure what is the best way for that part.
With blessings, thank you!