CHANGES:
- Skip unnecessary and missing unistd.h for MSVC compiler (@jonahbeckford, #66)
- Remove deprecated io-page.unix library (@hannesm, #67)
- Drop caml_ prefix from C symbols (@hannesm, #64)
- Remove bigarray-compat dependency (@hannesm, #67)
- Raise lower bound to OCaml 4.08.0 (@hannesm, #67)
- Make freestanding compilation possible without opam (@sternenseemann, #65)