Better Windows support

[LMP88959]

I don't plan on adding better Windows support nor do I have a good avenue to test Windows software so this issue is mainly for whoever is interested to work on.

The main issue is the lack of support for large input files due to 'long' being 32-bit on Windows.

This whole issue should amount to:

1. replacing 'long' with 'size_t' where appropriate in the file IO routines
2. (possibly) compiling with -D__FILE_OFFSET_BITS=64 as @notcancername suggested