[CMake] how to set current source directory relative to external project

[hex]

hello CMake community,


I am experimenting with external projects. I have documentation in an 
external project which is generated in `${CMAKE_CURRENT_SOURCE_DIR}`.


When I add the external project, however, it is using 
`${CMAKE_CURRENT_SOURCE_DIR}` of the "super build".


I need `${CMAKE_CURRENT_SOURCE_DIR}` to be equal to `${SOURCE_DIR}`, 
where the former is seen by the external project and the latter by the 
"super build".


Example:

message("${CMAKE_CURRENT_SOURCE_DIR} = /home/user/super")

ExternalProject_Add(ext1
     # directory options
     SOURCE_DIR      "${CMAKE_CURRENT_SOURCE_DIR}/ext1"
)


${CMAKE_CURRENT_SOURCE_DIR}/ext1/CMakeLists.txt:


message("${CMAKE_CURRENT_SOURCE_DIR} = /home/user/super/ext1")