I found the problem. My library folder name needs to match the library name. I had my library like this:
\Arduino\libraries\OneWireDallasTemp\DallasTemperature.h
\Arduino\libraries\OneWireDallasTemp\DallasTemperature.cpp
I renamed the folder \OneWireDallasTemp\ to \DallasTemperature\ and now it compiles. I guess the Arduino IDE doesn't care about the directory name but Xcode does.