CLI11 & GCC8 combo have special consideration #839

* add CLI11_HAS_FILESYSTEM=0

Author: alexsavulescu

CMake/config/CompilerFlagsHelpers.cmake

@@ -152,3 +152,12 @@ 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()