The runtime already detects if DirectX 9 is not installed in the DirectX builds. If the user doesn't have DX9 the following message is displayed:
"A required DirectX component, 'd3dx_30.dll', was not found. Please install the latest version of DirectX 9 in order to run this application.
Click 'OK' to visit the Microsoft to install the latest updates for DirectX 9.
Click 'Cancel' to terminate the application."
Clicking OK opens the user's browser to here
. It points to the DirectX download category because the original direct download link turned in to a 404 after a few builds, but there's a big link at the top for the latest end-user runtime.