revert [8abba84224], and make sure that every source file that uses Tcl_StatBuf has an "#include <sys/stat.h>" before including tcl.h
[Bug 3598300]: unix: tcl.h does not include sys/stat.h.
(with an exception for OSX, for now)
... and fix cygwin build
Proposed solution for Bug 3598300 on MacOSX