Fix cmake to link extra filesystem library with older versions of gcc and clang

This MR fixes build errors with older versions of the GCC and Clang compilers, which require linking an additional filesystem library (should also fix #16 (closed)).

The solution for Clang is not 100% waterproof because Clang can use different implementations of the standard library (libstdc++ from GCC or libc++ from LLVM itself). This fix chooses what is default for the supported platforms (Linux, Mac, Windows) but if non-standard configurations are used, this may fail.

This resolves #16 (closed)