Fix compilation in MSVC through CMake.

When using the CMake projects to compile, MSVC will show tons of errors. This is because it should treat the files as C++ code, not C code. (Because MSVC's C compiler is still in the stone-age.)

The whole point of CMake is to have only one "project file" for all systems. I am currently including Chipmunk in my project through CMake and it works on all platforms but MSVC. This fixes it.