Fix CMake for GLFW_Vulkan and add CMake for SDL_Vulkan on Linux

[ProfFan]

This fixes the GLFW and SDL2 Vulkan examples so they build cleanly on any modern Linux OS.

Should be helpful for anyone getting started in IMGUI.

[rokups]

Thank you for PR @ProfFan. This however breaks builds in multiple occasions. pkg-config is not available on windows if not using msys2 or similar environment so that would break example_glfw_vulkan build. Similarly example_sdl2_vulkan would not build with older cmake versions because FindVulkan was introduced in cmake v3.7 while build script requires only v3.0.

[ProfFan]

Hi @rokups , thanks for the suggestions! I can change the CMake to fallback to the original method if the new method does not succeed.

[smellymoo]

damn, which I checked here first, took me awhile to figure this out myself.

+1 for "Should be helpful for anyone getting started in IMGUI."