Add check for ruserok

ruserok is not available/functional in uclibc, provide conditions for compilation
where needed.

Signed-off-by: Dmitry Golubovsky <golubovsky@gmail.com>

diff -urN a/configure.in b/configure.in
--- a/configure.in	2012-08-17 03:48:24.000000000 -0500
+++ b/configure.in	2013-07-17 09:49:23.760254684 -0500
@@ -526,7 +526,7 @@
 AC_CHECK_FUNCS(strcspn strdup strspn strstr strtol uname)
 AC_CHECK_FUNCS(getutent_r getpwnam_r getpwuid_r getgrnam_r getgrgid_r getspnam_r)
 AC_CHECK_FUNCS(getgrouplist getline getdelim)
-AC_CHECK_FUNCS(inet_ntop inet_pton innetgr ruserok_af)
+AC_CHECK_FUNCS(inet_ntop inet_pton innetgr ruserok_af ruserok)
 
 AC_CHECK_FUNCS(unshare, [UNSHARE=yes], [UNSHARE=no])
 AM_CONDITIONAL([HAVE_UNSHARE], [test "$UNSHARE" = yes])