This applies to the current trunk r19895.

Right before the project() command in UseSlicer.cmake, it is checked whether the ${EXTENSION_NAME}_SOURCE_DIR is already defined, i.e., if the extension developer called project() prior to the include(UseSlicer.cmake).

This check can be disabled by setting Slicer_SKIP_USE_FILE_INCLUDE_CHECK. Another variable that allows to modify the code executed by UseSlicer.cmake is Slicer_SKIP_PROJECT_COMMAND. If this variable is set, but not Slicer_SKIP_USE_FILE_INCLUDE_CHECK, UseSlicer.cmake will yet raise a fatal error and ask the developer not to call project themselves. However, by setting Slicer_SKIP_PROJECT_COMMAND, the developer explicitly requested the UseSlicer.cmake file not to call project(), hence, the check should probably be skipped as well...

To be able to set Slicer_SKIP_USE_FILE_INCLUDE_CHECK without also setting Slicer_SKIP_PROJECT_COMMAND is reasonable and might be required in certain cases where this check gives a false alarm, yet the project() command should be invoked by UseSlicer.cmake. On the other side, if Slicer_SKIP_PROJECT_COMMAND is defined and TRUE, this should probably imply Slicer_SKIP_USE_FILE_INCLUDE_CHECK.

The "work-around" on the extension developer side is simple. Just set both Slicer_SKIP_PROJECT_COMMAND and Slicer_SKIP_USE_FILE_INCLUDE_CHECK to TRUE.

Find attached a patch of the proposed change.