Guard unistd.h includes and fix build on Windows

commit 7520a9df:

Guard unistd.h includes

commit 0873afc7:

fccompat: fix build on Windows without unistd.h
https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/access-waccess?view=vs-2019

CI pipeline with MSVC builds here is green as well.