Shader Fails to Compile on iOS: GL_EXT_frag_depth

I have a very simple masking shader (three, actually, that work in tandem.)

 Shader "Custom/CutoutA"
  {
      SubShader
      {
           Tags { "Queue" = "Geometry-3" }
           
          Pass
          {
              CGPROGRAM
              
              #pragma vertex vert
             #pragma fragment frag
 
             float4 vert(float4 v:POSITION) : SV_POSITION {
                 return mul (UNITY_MATRIX_MVP, v);
             }
             
              void frag(out float depth:DEPTH)
             {
                 depth = -1;
             }
              
              ENDCG
          }
      }
  }

They were working on my iOS device. I switched platforms to a Mac Standalone for a month in order to make a build to show on a tv at an event. I may have also updated from Unity 4.x to Unity 5.x. I can't remember exactly when that happened.

Recently, I've switched back to the mobile build. Upon compiling for device, these masking shaders are no longer working. They appear pink, so I

• placed the shaders in the Resources/ folder.

• added them to the Included Shaders section of the Graphics settings.

Neither helped.

Eventually I noticed that the runtime log (visible through Xcode while running on device) says the following:

 -------- Shader compilation failed
 #version 100
 #extension GL_EXT_frag_depth : enable
 void main ()
 {
   gl_FragDepthEXT = -1.0;
 }
 -------- failed compiling:
 fragment evaluation shader
 -------- GLSL error:
 
 WARNING: 0:4: extension 'GL_EXT_frag_depth' is not supported
 ERROR: 0:7: Use of undeclared identifier 'gl_FragDepthEXT'
 Note: Creation of internal variant of shader 'Custom/CutoutA' failed.
 WARNING: Shader Unsupported: 'Custom/CutoutA' - Setting to default shader.

Anyone have any experience with this? I've been able to view the generated shader code and it looks like it's trying to use the GL_EXT_frag_depth extension.