Is it difficult to turn games like Perigon that have been written in C++ for iPhone into games for PC?

It depends on how they were written. C++ is a cross-platform language. You can use it to write for any platform that has a C++ compiler developed for it.

The big question is whether or not OS-specific libraries are used by the program. If they were, you're likely to have to completely re-write the program using the API of an appropriate library for the target platform/OS.

If the code does not use OS-specific libraries, you should be able to simply re-compile it for a different target platform.

An application such as that though probably does use OS-Specific libraries.