utmpx.h is specified in POSIX but utmpxname is not so we check for
utmpxname in configure script.

This fixes the following compile error with musl libc:
lxcutmp.c: In function 'utmp_get_runlevel':
lxcutmp.c:249:2: error: implicit declaration of function 'utmpxname' [-Werror=implicit-function-declaration]
  if (!access(path, F_OK) && !utmpxname(path))
    ^
Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>