IK Look At script for Mecanim/Humanoid. Attach to your character. Before using it, you must turn on IK Pass of the Animator attached to your character.

IKLookAt.cs

//
// Mecanim.IkLookAt
// Mecanim.HumanoidでIK Look Atする。キャラクターにアタッチします。
// 使用する前にキャラクターにアタッチされているAnimatorのIK PassをONにしておくこと.
// 2017/01/14 N.Kobayashi
//
using UnityEngine;
using System.Collections;


namespace UnityChan
{
	[RequireComponent(typeof(Animator))]  
	public class IKLookAt : MonoBehaviour {

		protected Animator avatar;
		
		public bool ikActive = true;
        [HideInInspector]
        public GameObject cameraObject = null;
        [HideInInspector]
        public Transform lookAtObj = null;
        public bool isGUI = true;

        public float maxLookAtWeight = 1.0f;
		public float bodyWeight = 0.3f;
		public float headWeight = 0.8f;
		public float eyesWeight = 1.0f;
		public float clampWeight = 0.5f;
        private float lookAtWeight = 0.0f;

        public float minDistanceIkOn = 1.0f;
        public float maxDistanceIkOn = 3.0f;
        public float minDotIkOn = 0.5f;
        public Transform headJoint = null;
        public Transform forwardJoint = null;
        public float adjustSeconds = 2.0f;
        private float adjustDelta;
        private float sumAdjustDelta;
        private bool isLookAtEnable = false; 




        // Use this for initialization
        void Start () {
			avatar = GetComponent<Animator>();
            //lookAtObjectが指定されていない時には、MainCameraオブジェクトを探し、そのtransformをアタッチする.
            if (lookAtObj == null)
            {
                cameraObject = GameObject.FindGameObjectWithTag("MainCamera");
                lookAtObj = cameraObject.transform;
                //Debug.Log("Get MainCamera's Transform");
                
            }
        }

		void OnGUI()
		{
            if (isGUI)
            {
                GUILayout.Label("Activate Look at IK");
                ikActive = GUILayout.Toggle(ikActive, "Activate IK");
            }
		}


		void OnAnimatorIK(int layerIndex)
		{		
			if(avatar)
			{
				if(ikActive)
				{
                    if (isLookAtEnable)
                    {
                        avatar.SetLookAtWeight(lookAtWeight, bodyWeight, headWeight, eyesWeight, clampWeight);
                    }
                    if (lookAtObj != null)
                    {
                    	avatar.SetLookAtPosition(lookAtObj.position);
                    }
                }
				else
				{
					avatar.SetLookAtWeight(0.0f);
				}
			}
		}
		
		void Update () 
		{
			if(avatar)
			{
				if(ikActive)
				{
                    adjustDelta = Time.deltaTime / adjustSeconds;
                    //カメラとの距離と角度から、Look AtをON/OFFします.
                    Vector3 offsetVec = lookAtObj.position - headJoint.position;
                    Vector3 forwardVec = forwardJoint.position - headJoint.position;
                    float dist = offsetVec.sqrMagnitude;
                    float dot = Vector3.Dot(offsetVec.normalized, forwardVec.normalized);
                    if (((dist >= minDistanceIkOn * minDistanceIkOn) && (dist <= maxDistanceIkOn * maxDistanceIkOn)) && (dot >= minDotIkOn)){
                        isLookAtEnable = true;
                        //adjustDeltaの積算値よりIK Look Atのブレンド比率を変化させる.
                        if (sumAdjustDelta < 1.0f)
                        {
                            lookAtWeight = Mathf.SmoothStep(0.0f, maxLookAtWeight, sumAdjustDelta);
                            sumAdjustDelta = sumAdjustDelta + adjustDelta;
                            //Debug.Log("lookAtWeight : " + lookAtWeight);
                        }
                        else {
                            lookAtWeight = maxLookAtWeight;
                        }
                    } else {
                        isLookAtEnable = false;
                        sumAdjustDelta = 0.0f;
                        lookAtWeight = 0.0f;
                        //Debug.Log("lookAtWeight : Reset");
                    }
				}
			}		
		}   		  
	}
}