Avoid compiler complaints if _TIME_BITS defined when building zlib.

zlib does not use time_t, so _TIME_BITS is irrelevant. However it
may be defined anyway as part of a sledgehammer indiscriminately
applied to all builds.

gzguts.h

@@ -9,9 +9,8 @@
 #  ifndef _LARGEFILE_SOURCE
 #    define _LARGEFILE_SOURCE 1
 #  endif
-#  ifdef _FILE_OFFSET_BITS
-#    undef _FILE_OFFSET_BITS
-#  endif
+#  undef _FILE_OFFSET_BITS
+#  undef _TIME_BITS
 #endif
 
 #if defined(HAVE_VISIBILITY_INTERNAL)