Frist, sorry for the slow response to your question.  It looks like there could be a couple things going on. 

Its hard to say without looking at a sample but, some of the damage on the leaves looks like it could be bacterial leaf spot.  This is an issue that occurs when leaves are consistently wet.  The leaves look wet in the picture.  Is this from irrigation?  Is there a sprinkler that gets them wet regularly?  The spotting could also caused by salty water being sprayed on the plant's leaves.  This is only likely if you irrigate with well water or if your landscape is watered with water which comes from a separate system than your home's interior water (normally either reclamation water or again untreated water from a community well).  

Its also possible, as you suggest, that the landscape around the shrub and the way it was planted  have caused or contributed to the shrub stress and slowed its establishment.   The heat off the pavers and the compacted soil which is probably under them are the likely culprits.  However, lilacs are fairly tough plants and the symptoms don't look completely consistent with this explanation.

If there is a sprinkler spaying the plant see if you can adjust it. There is not much time left in the growing season but you might also consider watering the plant by hand right near its base for a while.   At the end of the season clean up all the leaves and put them out with trash (don't compost them on site).

I hope this helps, please let me know if you have further questions.