If you do decide to go with C++ there are a number of good cross-platform GUI libraries that will allow you to avoid having to duplicate the GUI code for each platform. For example:
There are a number of other similar projects but those are some of the better and more well-known ones. For the remainder of your code, anything system-specific will of course have to be written using separate C++ code to interface with Win32 APIs or OS X's system API where necessary. That being said, you may find you're able to avoid much of the system-specific code by using extensive libraries like Boost.
Other suggestions would be things like using a configuration file instead of the Windows registry or a plist file on the mac. Instead, shoot for platform-agnostic approaches wherever possible to minimize the places where you have to write code using system APIs.
