Add checks for vasprintf, localtime_r.

Add checks for vasprintf, localtime_r.

        * configure.in:
        Add checks for vasprintf, localtime_r.

        * gdate.c (g_date_set_time):
        Use localtime if localtime_r is not available.

        * gstrfuncs.c (g_strdup_vprintf):
        Use glibc vasprintf if possible; it's a bit faster than using
        GLib routines, and makes output code a bit smaller.

        * acconfig.h:
        Remove HAVE_VSNPRINTF and HAVE_VPRINTF.  autoheader picks these
        up automatically and puts them in config.h.in.