gettimeofday: fix C++ crosscompilation

gettimeofday: fix C++ crosscompilation

Never replace gmtime and localtime by macros when compiling with
C++, this prevents <ctime> from being included.

* m4/gettimeofday.m4 (gl_GETTIMEOFDAY_REPLACE_LOCALTIME): Do not
define gmtime and localtime as preprocessor macros.  Instead
define some HAVE_GMTIME, HAVE_LOCALTIME, REPLACE_GMTIME, and
REPLACE_LOCALTIME substitutions.
* lib/time.in.h: Declare gmtime and localtime when needed.
* m4/time_h.m4 (gl_HEADER_TIME_H_DEFAULTS): AC_SUBST HAVE_GMTIME,
HAVE_LOCALTIME, REPLACE_GMTIME, and REPLACE_LOCALTIME.
* modules/time: Depend on gettimeofday, and substitute the above
variables in time.h.