Enhance system sc and p4est linkage

[sandro-elsweijer]

Describe your changes here:
Enhances the system SC and p4est linkage in such a way, that the include path is also found and linked to the executables.
Furthermore, the output of cmake is enhanced so that is easier to see if something has gone wrong during linkage of external modules.

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As a reviewer please read through all the code lines and make sure that the code is fully understood, bug free, well-documented and well-structured.

General

• The reviewer executed the new code features at least once and checked the results manually

• The code follows the t8code coding guidelines

• New source/header files are properly added to the Makefiles

• The code is well documented

• All function declarations, structs/classes and their members have a proper doxygen documentation

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Tests

• The code is covered in an existing or new test case using Google Test

Github action

• The code compiles without warning in debugging and release mode, with and without MPI (this should be executed automatically in a github action)

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  If the Pull request introduces code that is not covered by the github action (for example coupling with a new library):

  • Should this use case be added to the github action?
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Scripts and Wiki

• If a new directory with source-files is added, it must be covered by the script/find_all_source_files.scp to check the indentation of these files.
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Licence

• The author added a BSD statement to doc/ (or already has one)

[jmark]

@sandro-elsweijer The tests are failing for the CMake testsuite.

[maelk3]

Shouldn't this be if(NOT P4EST_LIBRARIES) ?

[maelk3]

As it currently stands, the system SC and P4est libraries include path are already taken into consideration. As using find_package in config mode retrieves the SC-config.cmake files and doing target_link_libraries(T8 SC::SC) implies that all interface properties of the SC target are added to the T8 target (including include paths, compiler flags/definitions). Furthermore, the include path can be explicitly retrieved using get_target_property(T8_INCLUDE_DIR T8CODE::T8 INTERFACE_INCLUDE_DIRECTORIES). Having said that, I think the FindP4est.cmake and FindSC.cmake can be useful in case SC and P4est have not being installed with their respective CMake build system. However, we should favor using find_package in config mode as it is more robust and gets more information (like library version, necessary compiler flags...).