[cmake] define LINK_SUNDIALS_STATIC for static.

  - Add a new interface library sundials_interface_static that adds the
    define LINK_SUNDIALS_STATIC.

    It brings the include directories by linking to sundials_interface
    then adds the define.

    All libs that will link to static sundails libs will now get the
    define automatically.

    If we need to link to the shared sundails libs at any point just
    add a new interface library or use the sundials_interface library
    itself since it does not add the define.

CMakeLists.txt

@@ -200,16 +200,23 @@ target_include_directories(sundials_interface INTERFACE ${sundials_SOURCE_DIR}/i
 ## The sundials_config.h files are generated in the build directory. Add it as an include dir.
 target_include_directories(sundials_interface INTERFACE ${sundials_BINARY_DIR}/include/)
 
-## Now that the includes are attached to a utility interface library (sundials_interface) link it
-## to the sundials libs so they can be found when the sundials lib is linked-to from an external lib.
+## Add an interface lib for linking ot the static libs. This will transitively add
+## DLINK_SUNDIALS_STATIC to anything that links to the static sundials libs.
+add_library(sundials_interface_static INTERFACE)
+target_link_libraries(sundials_interface_static INTERFACE sundials_interface)
+target_compile_definitions(sundials_interface_static INTERFACE -DLINK_SUNDIALS_STATIC)
+
+## Now that the includes and defines are attached to a utility interface library (sundials_interface_static) link it
+## to the sundials static libs so they can be found when the sundials lib is linked-to from an external lib.
 ### Note! It should have been enough for the scope here to be INTERFACE instead of PUBLIC. However,
 ### that does not seem to work. This should be fine anyway.
-target_link_libraries(sundials_cvode_static PUBLIC sundials_interface)
-target_link_libraries(sundials_idas_static PUBLIC sundials_interface)
-target_link_libraries(sundials_kinsol_static PUBLIC sundials_interface)
-target_link_libraries(sundials_sunlinsolklu_static PUBLIC sundials_interface)
-target_link_libraries(sundials_sunlinsollapackdense_static PUBLIC sundials_interface)
+target_link_libraries(sundials_cvode_static PUBLIC sundials_interface_static)
+target_link_libraries(sundials_idas_static PUBLIC sundials_interface_static)
+target_link_libraries(sundials_kinsol_static PUBLIC sundials_interface_static)
+target_link_libraries(sundials_sunlinsolklu_static PUBLIC sundials_interface_static)
+target_link_libraries(sundials_sunlinsollapackdense_static PUBLIC sundials_interface_static)
 
+## Add aliases for the static libs. For readability.
 add_library(omc::3rd::sundials::cvode ALIAS sundials_cvode_static)
 add_library(omc::3rd::sundials::idas ALIAS sundials_idas_static)
 add_library(omc::3rd::sundials::kinsol ALIAS sundials_kinsol_static)