When compiling with the GDK the POSIX variants are defined but not available
for linking. We can just the Windows variants that are always available.
Do the proper redirection in our custom unistd.h used with the Platform SDK.
These functions are not available when linking, so we should avoid using them.
Also mingw-w64 understands _CRT_NONSTDC_NO_DEPRECATE but not
_CRT_NONSTDC_NO_WARNINGS.
Steve Lhomme