cmake: fix CMP0046 warnings in case doxygen is not found
cmake: fix CMP0046 warnings in case doxygen is not found

TelepathyIM/telepathy-qt,TelepathyIM/telepathy-qt,TelepathyIM/telepathy-qt,TelepathyQt/telepathy-qt,TelepathyIM/telepathy-qt,TelepathyIM/telepathy-qt,TelepathyQt/telepathy-qt,TelepathyQt/telepathy-qt,TelepathyQt/telepathy-qt