Make retrieval of file size more portable

[saxbophone]

Put sxbp_get_file_size() in a separate source file and used conditional compilation to provide three different implementations of it:

• A POSIX/UNIX version which uses POSIX fstat()
• A Windows version which uses GetFileSizeEx() (and associated other WinAPI functions)
• A generic version using just the C standard library (but which might not work, as it uses fseek() and SEEK_END, which isn't required to be supported meaningfully).

TODO:

• Test this on Windows
• Amend Windows version to return failure if file size > 2GiB on Win32 only, not Win64

[saxbophone]

I don't like this generic version. It does things in a fundamentally different way to the others. I think it should be removed instead.