The Constitution says that a person must be a natural born citizen of America to run for President, but obviously Washington, Adams, etc. weren't born in America, so I'm guessing there must be some caveat to that statement...
You will find the caveat in the exact same sentence of the constitution that says you must be a natural born citizen to become president.
Article 2, Section 1
No Person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any person be eligible to that Office who shall not have attained to the Age of thirty five Years, and been fourteen Years a Resident within the United States.