detect unistd.h availability

It doesn't exist on MSVC and it's not needed.
3 jobs for unistd in 1 minute and 11 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #1006
amd64 debian
build-debian

00:00:22

passed #1007
win32
build-win32

00:00:22

passed #1008
win64
build-win64

00:00:25