Simplify FetchContent for older cmake versions

.gitlab-ci.yml

 ---
 variables:
-  GIT_SUBMODULE_STRATEGY: recursive
   DUNECI_PARALLEL: 2
 
 .job_template: &job_configuration

libs/CMakeLists.txt

@@ -5,7 +5,12 @@ FetchContent_Declare(fmt                              # name of the content
   GIT_REPOSITORY https://github.com/fmtlib/fmt.git    # the repository
   GIT_TAG        9.1.0                                # the tag
   )
-FetchContent_MakeAvailable(fmt)
+
+FetchContent_GetProperties(fmt)
+if(NOT fmt_POPULATED)
+  FetchContent_Populate(fmt)
+  add_subdirectory(${fmt_SOURCE_DIR} ${fmt_BINARY_DIR})
+endif()
 install(TARGETS fmt EXPORT amdis-targets
           LIBRARY DESTINATION ${FMT_LIB_DIR}
           ARCHIVE DESTINATION ${FMT_LIB_DIR}