build: explicitly link stdc++fs on GCC 8

build-support/custom-modules/ns3-module-macros.cmake

@@ -104,7 +104,9 @@ function(build_lib)
       )
     endif()
 
-    if(NOT FILESYSTEM_LIBRARY_IS_LINKED)
+    if((NOT FILESYSTEM_LIBRARY_IS_LINKED) OR (${GCC} AND ${GCC8}))
+      # The GCC8 alternative is necessary since when installed alongside newer
+      # releases, the incorrect shared library can end up being linked.
       list(APPEND BLIB_LIBRARIES_TO_LINK -lstdc++fs)
     endif()
 

build-support/macros-and-definitions.cmake

@@ -202,6 +202,7 @@ if(CLANG)
 endif()
 
 set(GCC FALSE)
+set(GCC8 FALSE)
 if("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU")
   if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS ${GNU_MinVersion})
     message(
@@ -209,6 +210,12 @@ if("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU")
         "GNU ${CMAKE_CXX_COMPILER_VERSION} ${below_minimum_msg} ${GNU_MinVersion}"
     )
   endif()
+  if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS "9.0.0")
+    # This block is used to identify if GCC8 is being used. In this case, we
+    # want to explicitly link stdc++fs, which is done in
+    # ns3-module-macros.cmake.
+    set(GCC8 TRUE)
+  endif()
   set(GCC TRUE)
   add_definitions(-fno-semantic-interposition)
   if(${NS3_COLORED_OUTPUT} OR "$ENV{CLICOLOR}")