PAHO_WITH_MQTT_C OFF export error

[ryanjmulder]

When I set PAHO_WITH_MQTT_C to OFF, I get the cmake error:

export called with target "paho-mqttpp3-static" which requires target
[cmake]   "paho-mqtt3as-static" that is not in any export set.

It seems to be because these lines only happen in the PAHO_WITH_MQTT_C block

## install paho.mqtt.c library (appending to PahoMqttCpp export)
    install(TARGETS ${PAHO_MQTT_C_LIBS}
        EXPORT PahoMqttCpp
        ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
        LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
        RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
    )