GCC and Clang provide the "-m32" and "-m64" options, which tell the compiler to build as 32 bit and 64 bit, respectively.
Unfortunately MSVC doesn't provide similar options, thus this commit converts "overlay" from a subdirectory to a project.
If the build is 32 bit, the project is processed a single time and compiles a 32 bit library.
If the build is 64 bit, the project is processed two times and compiles a 64 bit and a 32 bit library.
