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.

Source/kwsys/SystemTools.cxx

@@ -64,9 +64,12 @@
 #include <signal.h>    /* sigprocmask */
 #endif
 
-// Windows API.  Some parts used even on cygwin.
-#if defined(_WIN32) || defined (__CYGWIN__)
+// Windows API.
+#if defined(_WIN32)
+# include <windows.h>
+#elif defined (__CYGWIN__)
 # include <windows.h>
+# undef _WIN32
 #endif
 
 #ifdef __CYGWIN__