link stdc++fs when using gcc8

CMakeLists.txt

@@ -80,6 +80,13 @@ if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
     message (FATAL_ERROR ">> The ${CMAKE_CXX_COMPILER_ID} compiler version ${CMAKE_CXX_COMPILER_VERSION} is unsupported, requires at least version ${OB_COMPILER_VERSION_GNU}")
   endif ()
 
+  if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS "9.0.0")
+    set (OB_LINK_LIBRARIES
+      ${OB_LINK_LIBRARIES}
+      stdc++fs
+    )
+  endif ()
+
   set (OB_FLAGS_GENERAL "-std=c++17 -fdiagnostics-color=auto -fmax-errors=8")
 
   set (OB_FLAGS_DEBUG "-g -Og -rdynamic -Wpedantic -Wall -Wextra -Wcast-align -Wcast-qual -Wctor-dtor-privacy -Wdisabled-optimization -Wformat=2 -Winit-self -Wlogical-op -Wmissing-declarations -Wmissing-include-dirs -Wnoexcept -Wold-style-cast -Woverloaded-virtual -Wredundant-decls -Wshadow -Wsign-conversion -Wsign-promo -Wstrict-null-sentinel -Wstrict-overflow=5 -Wswitch-default -Wundef -Wno-unused")