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.
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