Add comment and check for SHADER_TARGET_AVAILABLE

ScriptableRenderPipeline/Core/CoreRP/ShaderLibrary/Common.hlsl

 
 #endif // #ifndef real
 
+// Target in compute shader are supported in 2018.2, for now define ours
+// (Note only 45 and above support compute shader)
-#define SHADER_TARGET 50 // Not defined otherwise
+#   ifndef SHADER_TARGET
+#       if defined(SHADER_API_METAL) || defined(SHADER_API_VULKAN)
+#       define SHADER_TARGET 45
+#       else
+#       define SHADER_TARGET 50
+#       endif
+#   endif
 #endif
 
 // Include language header