mktime: sync from glibc

mktime: sync from glibc

* config/srclist.txt: Uncomment mktime.c.
* lib/mktime.c: Sync from glibc master.  This incorporates 3 changes.
First, indent with tabs, since glibc uses tabs and doesn't want to
change and we'd rather be identical to glibc.  Also, two small
coding changes:
(isdst_differ): Use &&, not &, as && is the usual style.
(__mktime_internal): Rename local var from abs_diff to approx_abs_diff
for clarity.