Change Shademode to get Data from one provoking vertex

The closer I got to, which doesn't work on mac is

 Shader "Flattener" {
     SubShader {
         Pass {
             GLSLPROGRAM
 
             #extension GL_EXT_gpu_shader4 : require
             flat varying vec4 color;
 
             #ifdef VERTEX
             void main()
             {
                 color = gl_Color;
                 gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
             }
             #endif
 
             #ifdef FRAGMENT
             void main()
             {
                 gl_FragColor = color; // set the output fragment color
             }
             #endif
 
             ENDGLSL
         }
     }
 }