Fix Wundef warning for MEMCPY_OK_FOR_FWD_MEMMOVE

Define MEMCPY_OK_FOR_FWD_MEMMOVE in memcopy.h and let arch-specific
implementations of that file override the value if necessary.  This
override is only useful for tile and moving this macro to memcopy.h
allows us to remove the tile-specific memmove.c.
Sync up mktime with gnulib

From the gnulib commit log:

commit e2646b0c6b5acda25e9ffeb4c12a5513a1e3b5ac
Author: Paul Eggert <eggert@cs.ucla.edu>
Date:   Fri Jun 27 11:35:44 2014 -0700

    mktime: merge #if/#ifdef usage from glibc

    * lib/mktime.c: Use "#if defined DEBUG && DEBUG", not "#if DEBUG",
    as that works with both Glibc's and Gnulib's style.
    See thread starting at Siddhesh Poyarekar's bug report at:
    http://lists.gnu.org/archive/html/bug-gnulib/2014-06/msg00102.html