Check if we need to link against stdc++fs (#840)

* https://en.cppreference.com/w/cpp/filesystem#Notes
* Revert "CLI11 & GCC8 combo have special consideration  #839"
  * This reverts commit 3a27464.
* no clang check for now

Author: alexsavulescu

CMake/config/CompilerFlagsHelpers.cmake

@@ -152,12 +152,3 @@ if(CMAKE_CXX_COMPILER_ID MATCHES "AppleClang" OR ${CMAKE_SYSTEM_NAME} MATCHES "D
   string(APPEND CMAKE_SHARED_LIBRARY_CREATE_CXX_FLAGS " ${UNDEFINED_SYMBOLS_IGNORE_FLAG}")
   string(APPEND CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS " ${UNDEFINED_SYMBOLS_IGNORE_FLAG}")
 endif()
-
-# ===============================================================================
-# See "Note: Special instructions for GCC 8" on https://github.com/CLIUtils/CLI11
-# ===============================================================================
-if(CMAKE_CXX_COMPILER_IS_GCC
-   AND ${CMAKE_CXX_COMPILER_VERSION} VERSION_GREATER_EQUAL 8
-   AND ${CMAKE_CXX_COMPILER_VERSION} VERSION_LESS 9)
-  add_compile_definitions(CLI11_HAS_FILESYSTEM=0)
-endif()

coreneuron/CMakeLists.txt

@@ -260,6 +260,11 @@ target_include_directories(coreneuron SYSTEM
 target_include_directories(coreneuron SYSTEM
                            PRIVATE ${CORENEURON_PROJECT_SOURCE_DIR}/external/CLI11/include)
 
+# See: https://en.cppreference.com/w/cpp/filesystem#Notes
+if(CMAKE_CXX_COMPILER_IS_GCC AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 9.1)
+  target_link_libraries(coreneuron stdc++fs)
+endif()
+
 if(CORENRN_ENABLE_GPU)
   # nrnran123.cpp possibly-temporarily uses Boost.Pool in GPU builds if it's available.
   find_package(Boost QUIET)