Add warning if system libusb is being added.
diff --git a/configure.ac b/configure.ac
index 12a900c..a32a066 100644
--- a/configure.ac
+++ b/configure.ac
@@ -565,3 +565,7 @@ echo
echo "Installation...........: make install (as root if needed, with 'su' or 'sudo')"
echo " prefix...............: $prefix"
echo
+if test "x$dlibusb" != xno; then
+echo "*** SYSTEM LIBUSB BEING ADDED - NOT RECOMMENDED UNLESS YOU ARE UNABLE TO COMPILE THE INCLUDED LIBUSB ***"
+echo
+fi