I second the vote for the third option. Not only is it the easiest option, but it keeps the benefits of of the C99 code for the platforms that support it. It is rather sad that, despite being standardized in 1999/2000, that it still has so little support. In fact, the toughest sticking point my team faced when it decided whether or not to adopt Chipmunk was that the C99 features would limit the platforms the game would be portable to. Not that we were planning on porting the game to more than OS X/Windows/Linux/BSD, but since we were going with pretty standard C code, it would have been trivial to port it to, for example, the DS or PSP.
Although I have next to no experience with MSVC, Chipmunk compiles without any changes under MinGW. Unfortunately, the MinGW DLL files are not compatible with the MSVC DLL files. Using MSVC's lib.exe cool, it is possible to create a chipmunk.lib which, I have been told, will allow the MSVC project to statically link to the MinGW dll. What I am not sure of, however, is whether this allows the DLL to be used with other languages, such as C#, which I am told that DLLs compiled with MSVC can do.
I may be wrong on some of the details, as I am not a Windows person. I do know that Chipmunk compiles without any changes using MinGW, and that the DLL files are not compatible across compilers.