BeOS portability.

BeOS portability.
* src/uptime.c: Include OS.h if it exists.
(print_uptime): On BeOS, use the get_system_info function (actually a
macro). Loop through utmp entries only if utmp.h or utmpx.h exists.
(uptime): Call read_utmp only if utmp.h or utmpx.h exists.
* m4/boottime.m4 (GNULIB_BOOT_TIME): Test also for utmp.h, utmpx.h,
OS.h. Don't require to have utmp.h or utmpx.h. Enable boottime
support if <OS.h> is found.