Ted Cruz was born in Alberta, Canada to an American mother and a Cuban father. I define a natural-born citizen of the United States as someone who obtains citizenship at birth. By having an American born mother, Ted Cruz fulfills this requirement and is eligible for the presidency.
As a side note, by my logic, even if President Obama had been born in Kenya (even though he was not), he would still be eligible for election because of his American born mother.
The Constitution doesn't clearly define this. But let's assume it means someone who is legally American by birth. Ted Cruz was born to an American mother so he inherits her citizenship. There hasn't been any landmark Supreme Court Case that determines what exactly natural born is but if we define it as someone who is American by birthright, he is. It says nowhere in the Constitution that a President must be born in America and legal experts agree that he's eligible.
Ted Cruz was not born in America. He was born in Calgary, Canada. I think everyone will agree that the President of the most powerful nation must be born in that nation, just as President Obama. Ted Cruz was declared Canadian first - of this there is no argument. It's all right there in the birth certificate he released this year - the birth certificate from Canada, NOT the USA. So he is a Canadian first and a President must be American first, like President Obama, who unlike Cruz was born in Hawaii, USA and not Calgary, Canada like Ted Cruz.