cmake: when building, tell people if serial support exists
cmake: when building, tell people if serial support exists

If we found libserialport (or didn't find it) tell people.

Signed-off-by: Robin Getz <c66e5a86ef9317cf35cecb2ed063996b2100ed4b@analog.com>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio