- Update premake build script for VS2017+ build support.

- Update premake (for VS2019 support).
- Update Xenia build script to detect VS2017+ version.
- Update Xenia premake scripts due to updated premake.
- Fix cc override in Xenia build script.

src/xenia/ui/vulkan/premake5.lua

@@ -37,9 +37,6 @@ project("xenia-ui-window-vulkan-demo")
     "xenia-ui-spirv",
     "xenia-ui-vulkan",
   })
-  flags({
-    "WinMain",  -- Use WinMain instead of main.
-  })
   defines({
   })
   includedirs({