The flags F_OK, R_OK and W_OK are defined in unistd.h, which does not exist under the Windows platform. Windows still defines the access function. Its access flags are documented at https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/access-waccess. For compatibility, define the flags F_OK, R_OK and W_OK using these constants.
This patch is part of the series to make flang compilable with MS Visual Studio http://lists.llvm.org/pipermail/flang-dev/2020-July/000448.html.