cygwin: patch vulkan headers for VKAPI_CALL and enable vulkan in cmake

CMakeLists.txt

@@ -378,7 +378,7 @@ dep_option(SDL_RENDER_D3D12        "Enable the Direct3D 12 render driver" ON "SD
 dep_option(SDL_RENDER_METAL        "Enable the Metal render driver" ON "SDL_RENDER;APPLE" OFF)
 dep_option(SDL_RENDER_GPU          "Enable the SDL_GPU render driver" ON "SDL_RENDER;SDL_GPU" OFF)
 dep_option(SDL_VIVANTE             "Use Vivante EGL video driver" ON "${UNIX_SYS};SDL_CPU_ARM32" OFF)
-dep_option(SDL_VULKAN              "Enable Vulkan support" "${SDL_VULKAN_DEFAULT}" "SDL_VIDEO;ANDROID OR APPLE OR LINUX OR FREEBSD OR OPENBSD OR WINDOWS" OFF)
+dep_option(SDL_VULKAN              "Enable Vulkan support" "${SDL_VULKAN_DEFAULT}" "SDL_VIDEO;ANDROID OR APPLE OR LINUX OR FREEBSD OR OPENBSD OR WINDOWS OR CYGWIN" OFF)
 dep_option(SDL_RENDER_VULKAN       "Enable the Vulkan render driver" ON "SDL_RENDER;SDL_VULKAN" OFF)
 dep_option(SDL_METAL               "Enable Metal support" ON "APPLE" OFF)
 set_option(SDL_OPENVR              "Use OpenVR video driver" OFF)

src/video/khronos/vulkan/vk_platform.h

@@ -36,7 +36,7 @@ extern "C"
  * Function declaration:  VKAPI_ATTR void VKAPI_CALL vkCommand(void);
  * Function pointer type: typedef void (VKAPI_PTR *PFN_vkCommand)(void);
  */
-#if defined(_WIN32)
+#if defined(_WIN32) || defined(__CYGWIN__) /* __CYGWIN__ added by SDL */
     // On Windows, Vulkan commands use the stdcall convention
     #define VKAPI_ATTR
     #define VKAPI_CALL __stdcall