The interface is between pipeline stages, fragment shader would not have an output interface because there is no exposed pipeline stage after the fragment shader. With that said the fragment shader takes input from previous pipeline stages and as such the input in your case from the vertex shader ( you are declaring out gl_PerVertex ) must match the input the fragment shader is expecting
As per in vec4 gl_FragCoord... the gl_FragCoord builtin is NOT an input for the fragment shader..it is an output.
Finally from the looks of things and as explained before..this does not seems to be an AMD error or a driver issue so far. How is the error meaningless ?