Remove the dependency of the strftime module on the tzset module.

Remove the dependency of the strftime module on the tzset module.
* modules/strftime (Depends-on): Remove dependency on tzset.
* lib/strftime.c (my_strftime) [! defined _LIBC && ! HAVE_RUN_TZSET_TEST]:
Copy the input structure, to work around some of the bug with
Solaris 2.5.1 and Solaris 2.6.  If you still care about these old
Solaris releases, you should also use the tzset module, but we won't
require it as a dependency any more since we don't want LGPLed code
to depend on GPLed code.
* m4/strftime.m4 (gl_FUNC_STRFTIME): Don't require gl_FUNC_TZSET_CLOBBER.