configure.ac: skip /dev/urandom check when cross-compiling

                 [read randomness from FILE (default=/dev/urandom)]),

      [ RANDOM_FILE="$withval" ],

      [

          if test x$cross_compiling != xyes; then

            dnl Check for random device

            AC_CHECK_FILE("/dev/urandom", [ RANDOM_FILE="/dev/urandom"] )

          else

            AC_MSG_WARN([skipped the /dev/urandom detection when cross-compiling])

          fi

      ]

  )

  if test -n "$RANDOM_FILE" && test X"$RANDOM_FILE" != Xno ; then