This seems like it may come up as an issue of interest. I don't think it really amounts to anything, but here's some previous writing on it.
McCain was indeed born in the Canal Zone, and Article II of the Constitution plainly states that "no person except a natural born Citizen... shall be eligible to the Office of President."
Some might define the term "natural-born citizen" as one who was born on United States soil. But the First Congress, on March 26, 1790, approved an act that declared, "The children of citizens of the United States that may be born beyond sea, or outside the limits of the United States, shall be considered as natural-born citizens of the United States." That would seem to include McCain, whose parents were both citizens and whose father was a Navy officer stationed at the U.S. naval base in Panama at the time of John's birth in 1936.
I don't really remember it but apparently there was a dispute about McCain's citizenship due to his birth in the Panama Canal Zone. Everything old is new again.