Improved handing of external rabbitmq-codegen
Improved handing of external rabbitmq-codegen

- By default look in the CMAKE_SOURCE_DIR/rabbitmq-codegen, then
CMAKE_SOURCE_DIR/../rabbitmq-codegen unless RABBITMQ_CODEGEN_DIR is set
- If FETCH_CODEGEN_FROM_GIT use the CMake ExternalProject to do it

jqk6/rabbitmq-c,Haster2004/rabbitmq-c,soindy/rabbitmq-c,sakishum/rabbitmq-c,hjzljp/rabbitmq-c,soindy/rabbitmq-c,alanxz/rabbitmq-c,jasonmsoft/rabbitmq-c,sakishum/rabbitmq-c,soindy/rabbitmq-c,alanxz/rabbitmq-c,hjzljp/rabbitmq-c,alanxz/rabbitmq-c,kilinrax/rabbitmq-c,tempbottle/rabbitmq-c,tempbottle/rabbitmq-c,Haster2004/rabbitmq-c,hjzljp/rabbitmq-c,chenwenbin928/rabbitmq-c,kilinrax/rabbitmq-c,chenwenbin928/rabbitmq-c,jasonmsoft/rabbitmq-c,jqk6/rabbitmq-c,jqk6/rabbitmq-c,jqk6/rabbitmq-c,chenwenbin928/rabbitmq-c,jasonmsoft/rabbitmq-c,jasonmsoft/rabbitmq-c,kilinrax/rabbitmq-c,tempbottle/rabbitmq-c,sakishum/rabbitmq-c,tempbottle/rabbitmq-c,hjzljp/rabbitmq-c,Haster2004/rabbitmq-c