Only link libstdc++fs for gcc8

https://discourse.cmake.org/t/correct-way-to-link-std-filesystem-with-gcc-8/4121/6

CMakeLists.txt

@@ -11,7 +11,7 @@ set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_INCLUDE_CURRENT_DIR ON)
 
 project(triton)
-link_libraries("-lstdc++fs")
+link_libraries( "$<$<AND:$<CXX_COMPILER_ID:GNU>,$<VERSION_LESS:$<CXX_COMPILER_VERSION>,9.0>>:-lstdc++fs>" )
 include(CTest)
 if(NOT WIN32)
   list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")