Okay, double check that you are configuring cmake appropriately, specifically setting the MAKE_INSTALL_PREFIX correctly.

mkdir build
cd build
cmake -DCMAKE_INSTALL_PREFIX=/usr ../

Also, it looks like you are only copying the xml file when doing a release build in your CMakeLists.txt:
install(FILES ${CMAKE_CURRENT_BINARY_DIR}/indi_rpicam2x.xml CONFIGURATIONS Release DESTINATION ${INDI_DATA_DIR})

Try changing that line to
install(
    FILES
    ${CMAKE_CURRENT_BINARY_DIR}/indi_rpicam2x.xml
    DESTINATION ${INDI_DATA_DIR}
)


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