Check for _getopt_long_only_r and force using included version of getopt if the function is not available.

configure.ac

@@ -262,6 +262,14 @@ AC_CHECK_TYPE(iconv_t,:,
 #endif
 ])
 
+# Check if we have reentrant versions of getopt. Argp needs them,
+# but many versions of Glibc lack them, though they define the
+# same getopt interface version as lib/getopt.c.
+AC_CHECK_FUNC(_getopt_long_only_r,
+              :,
+              [AC_DEFINE(WANT_INCLUDED_GETOPT,1,
+                         [Define if the included version of getopt has to be used])])
+
 AC_SUBST(BACKUP_LIBEXEC_SCRIPTS)
 AC_SUBST(BACKUP_SBIN_SCRIPTS)
 AC_ARG_ENABLE(backup-scripts,