Restore KWSys SystemTools _WIN32 state on cygwin

The commit "Fix KWSys SystemTools build on cygwin with -mwin32" tried to
restore the state of the _WIN32 definition that was broken by the commit
"Optimize KWSys SystemTools::FileExists on Windows".  It did so for the
case of building with -mwin32 on cygwin, but since including <windows.h>
defines _WIN32, it failed for the case of not using -mwin32.

This commit restores the state of _WIN32 in all cases by undefining it
after including <windows.h> if it was not defined beforehand.