Use PROJECT_SOURCE_DIR instead of CMAKE_SOURCE_DIR to allow for better submodulization

[apozharski]

When using cmake add_subdirectory if using acados as a submodule, the use of CMAKE_SOURCE_DIR adds the wrong directories. Using PROJECT_SOURCE_DIR instead produces the same results when building acados on its own but also adds the correct directories when included in other CMAKE projects.

[FreyJo]

Makes sense, thanks!

[imciner2]

I don't think the PROJECT_SOURCE_DIR would work if the upper CMake has a project() call in it though, then it would just add the root project directory instead of the acados directory, see the docs https://cmake.org/cmake/help/latest/variable/PROJECT_SOURCE_DIR.html that say

This is the source directory of the last call to the project() command made in the current directory scope or one of its parents. Note, it is not affected by calls to project() made within a child directory scope (i.e. from within a call to add_subdirectory() from the current scope).

Instead, it should be ${CMAKE_CURRENT_SOURCE_DIR}/../ in the path to add these acados folders.