I strongly suggest every cocos2d v2.x developer to upgrade to this version rather than applying the quick fix below. There are many 64-Bit compatibility issues lurking in cocos2d v2.0 and v2.1, which may rear their ugly head in weird bugs, visual glitches or more compile errors - depending on the app.
Beginning in February 2015 Apple will only accept new apps and app upgrades which include 64-Bit binaries. So it's highly recommended to get your code and cocos2d to a proper working state on 64-bit devices.
Quick & dirty fix only for this particular issue:
Edit the line the compiler complains about, change
float so that it reads:
extern float __ccContentScaleFactor;
However this indicates that xcode is building the arm64 slice. For cocos2d-iphone v2.x it is best to disable/remove arm64 from supported architectures because Cocos2D v2.1 and earlier are not fully compatible with 64bit devices!
There can be weird bugs and side-effects when running a Cocos2D v2.1 or earlier app compiled as 64-bit code and running on a 64-Bit device (iPhone 5S and newer). These issues may not occur on the iOS Simulator.