Fix timezone management in the C library

Define 'timezone' and 'daylight' global variables that are already
defined in <time.h>

Properly update the 'tm_gmtoff' field in 'struct tm' values.
diff --git a/libc/docs/CHANGES.TXT b/libc/docs/CHANGES.TXT
index d7f54ba..c053714 100644
--- a/libc/docs/CHANGES.TXT
+++ b/libc/docs/CHANGES.TXT
@@ -70,6 +70,12 @@
 
 - add sigaltstack() implementation for ARM.
 
+- <time.h>: Properly implement the 'timezone' and 'daylight' global variables
+  (they were not defined previously, though declared in the header).
+
+- <time.h>: Fix timezone management implementation to properly update
+  'tm_gmtoff' field in 'struct tm' structure.
+
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