See
https://sourceforge.net/tracker/?func=detail&atid=310894&aid=656765&group_id=10894
Fixes
../generic/tclClock.c: In function `FormatClock':
../generic/tclClock.c:314: warning: assignment makes integer from pointer without a cast
../generic/tclClock.c:315: error: invalid lvalue in assignment
../generic/tclClock.c:360: error: invalid lvalue in assignment
make: *** [tclClock.o] Error 1
when building tcl-8.4.5 under cygwin.

--- tcl8.4.5/generic/tclClock.c.old	2004-03-18 14:05:05.000000000 -0800
+++ tcl8.4.5/generic/tclClock.c	2004-03-18 14:08:06.000000000 -0800
@@ -311,8 +311,13 @@
             savedTZEnv = NULL;
 	}
         Tcl_SetVar2(interp, "env", "TZ", "GMT", TCL_GLOBAL_ONLY);
+#if !defined(__CYGWIN__)
         savedTimeZone = timezone;
         timezone = 0;
+#else 
+        savedTimeZone = _timezone;
+        _timezone = 0;
+#endif 
         tzset();
     }
 #endif
@@ -357,7 +362,11 @@
         } else {
             Tcl_UnsetVar2(interp, "env", "TZ", TCL_GLOBAL_ONLY);
         }
+#if !defined(__CYGWIN__)
         timezone = savedTimeZone;
+#else 
+        _timezone = savedTimeZone;
+#endif
         tzset();
     }
     Tcl_MutexUnlock( &clockMutex );