Was the 2008 US raid on the construction site in Abu Kamal in Syria which is neutral territory lawful under US law?

Yes. The President of the United States has the power under the War Powers Act of 1973 to wage war for up to 60 days without consent of congress, as long as congress is notified within 48 hours that said war is being waged. Additionally, the Authorization for Use Of Military Force Against Terrorists signed on September 18, 2001 grants the president the authority to pursue al-Qaeda and anyone who aids or harbors them (ie: the Taliban, the many associated terrorist groups that are often incorrectly referred to as "al-Qaeda", and entities who contribute to al-Qaeda or any other entity supporting al-Qaeda)

Additionally, subsequent reports have indicated that the Syrian government may have been complicit in the raid. For more information on the raid: http://en.wikipedia.org/wiki/2008_Abu_Kamal_raid