After all Subshaders a Fallback can be defined. It basically says “if none of subshaders can run on this hardware, try using the ones from another shader”.
    Fallback "name"
Fallback to shader with a given name or…
    Fallback Off
Explicitly state that there is no fallback and no warning should be printed, even if no subshaders can run on this hardware.
A fallback statement has the same effect as if all subshaders from the other shader would be inserted into its place.
    Shader "example" {
        // properties and subshaders here...
        Fallback "otherexample"
    }