CMake: consider removing deal_ii_setup_target() and using import targets directly (deal.II version 9.5 or later)

[tamiko]

There is nothing wrong with the current setup of using deal_ii_setup_target() in aspect. (In particular modifying the various DEAL_II_* variables for compiler and linker flags and then simply calling deal_ii_setup_target() is very convenient)

In deal.II 9.5 or later we switched to the modern way of using CMake import targets (effectively deprecating the deal_ii_setup_target() macro). This is particularly convenient if you ever want to take full control of compiler and linker setup independently of the configuration originally chosen for deal.II. On the flip-side this requires deal.II version 9.5 or newer.

Changing the CMake configuration would look as follows:

• instead of using deal_ii_setup_target(<target> DEBUG|RELEASE) simply use the CMake builtin target_link_libraries(<target> dealii::dealii_debug|release).

• Set up CMAKE_(C|CXX|Fortran)_FLAGS(|_DEBUG|_RELEASE) (or ASPECT_, or whatever) and compile definitions and populate them according to your needs and write a small wrapper that sets those flags for targets.