Does not compile properly on 5.4.0f3

Wanted to give this a try in a setup on 5.4.0f3, sadly the shaders don't even compile right.

```
Shader error in 'Custom/StandardClippable': 'MainLight': no matching 0 parameter function at Clipping/Shaders/standard_clipped.cginc(181) (on d3d11)

Compiling Vertex program with DIRECTIONAL LIGHTMAP_OFF DYNAMICLIGHTMAP_OFF
Platform defines: UNITY_ENABLE_REFLECTION_BUFFERS UNITY_PBS_USE_BRDF1 UNITY_SPECCUBE_BOX_PROJECTION UNITY_SPECCUBE_BLENDING
```

```
Shader error in 'Custom/StandardClippable': invalid subscript 'smoothness' at Clipping/Shaders/standard_clipped.cginc(188) (on d3d11)

Compiling Vertex program with DIRECTIONAL LIGHTMAP_OFF DYNAMICLIGHTMAP_OFF
Platform defines: UNITY_ENABLE_REFLECTION_BUFFERS UNITY_PBS_USE_BRDF1 UNITY_SPECCUBE_BOX_PROJECTION UNITY_SPECCUBE_BLENDING
```

Test project used forward rendering path.

Reproduce the issue in a Unity 5.4.0f3 project using the forward rendering path and D3D11. Read Clipping/Shaders/standard_clipped.cginc around lines 181 and 188, then verify the Custom/StandardClippable shader compiles without the reported MainLight and smoothness errors.

Written by the indexing model from the issue text.