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

Bug: http://curl.haxx.se/bug/view.cgi?id=3307835

configure.ac

@@ -1624,8 +1624,12 @@ if test X"$OPENSSL_ENABLED" = X"1"; then
                  [read randomness from FILE (default=/dev/urandom)]),
       [ RANDOM_FILE="$withval" ],
       [
-          dnl Check for random device
-          AC_CHECK_FILE("/dev/urandom", [ RANDOM_FILE="/dev/urandom"] )
+          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