LW: Keep master compiling with 2018.1

ScriptableRenderPipeline/LightweightPipeline/LWRP/Editor/ShaderPreprocessor.cs

 
 namespace UnityEditor.Experimental.Rendering.LightweightPipeline
 {
+#if UNITY_2018_2_OR_NEWER
     public class ShaderPreprocessor : IPreprocessShaders
     {
 #if LOG_VARIANTS
 #endif
         }
     }
+#endif // UNITY_2018_2_OR_NEWER
 }

ScriptableRenderPipeline/LightweightPipeline/LWRP/LightweightPipelineCore.cs

         public static readonly string SoftShadowsText = "_SHADOWS_SOFT";
         public static readonly string CascadeShadowsText = "_SHADOWS_CASCADE";
 
+#if UNITY_2018_2_OR_NEWER
         public static readonly ShaderKeyword AdditionalLights = new ShaderKeyword(AdditionalLightsText);
         public static readonly ShaderKeyword VertexLights = new ShaderKeyword(VertexLightsText);
         public static readonly ShaderKeyword MixedLightingSubtractive = new ShaderKeyword(MixedLightingSubtractiveText);
 
         public static readonly ShaderKeyword Lightmap = new ShaderKeyword("LIGHTMAP_ON");
         public static readonly ShaderKeyword DirectionalLightmap = new ShaderKeyword("DIRLIGHTMAP_COMBINED");
+#endif
     }
 
     public partial class LightweightPipeline