readutmp, boot-time: Fix compilation error on Android API 8. Reported by Po Lu in <https://lists.gnu.org/archive/html/bug-gnulib/2023-08/msg00108.html>. * m4/readutmp.m4 (gl_PREREQ_READUTMP_H): Test whether sysinfo is declared. * lib/boot-time-aux.h (get_linux_uptime): Invoke sysinfo only if it is declared. * doc/glibc-functions/sysinfo.texi: Mention the Android problem.