CMake: Add a user message when requesting to build the ATLASExtensions without having cloned the submodule
When enabling the build of the ATLASExtensions submodule by using the -DGEOMODEL_BUILD_ATLASEXTENSIONS=1
CMake option, there's no check whether the submodule has been cloned or not. So, the general user can be confused by the very short message given by CMake by default.
So, I have added a check in the CMake configuration: now, the presence of a CMakeLists.txt
file is verified when the user asks for the build of the ATLAS extensions. If not found, then a message with an explanation is given to the user, together with the command to run to clone the submodule: