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)