added cmake code to automatically update git submodules

externals/CMakeLists.txt

+find_package(Git QUIET)
+if (GIT_FOUND AND EXISTS "${PROJECT_SOURCE_DIR}/.git")
+# Update submodules as needed
+  option(GIT_SUBMODULE "Check submodules during build" ON)
+  if (GIT_SUBMODULE)
+    execute_process(COMMAND ${GIT_EXECUTABLE} submodule update --init --recursive
+                    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
+                    RESULT_VARIABLE GIT_SUBMOD_RESULT)
+    if(NOT GIT_SUBMOD_RESULT EQUAL "0")
+      message(FATAL_ERROR "git submodule update --init failed with ${GIT_SUBMOD_RESULT}, please checkout submodules")
+    endif()
+  endif()
+endif()
+
+if (NOT EXISTS "${PROJECT_SOURCE_DIR}/externals/fmt/CMakeLists.txt")
+  message(FATAL_ERROR "The submodules were not downloaded! GIT_SUBMODULE was turned off or failed. Please update submodules and try again.")
+endif()
+
+
 dune_library_add_sources(fmt SOURCES
   fmt/src/format.cc
   fmt/src/posix.cc