build: bump minimum GCC version to 9.1 and remove libfs compiler workaround

build-support/custom-modules/ns3-compiler-workarounds.cmake

@@ -14,22 +14,3 @@
 # Place, Suite 330, Boston, MA  02111-1307 USA
 #
 # Author: Gabriel Ferreira <gabrielcarvfer@gmail.com>
-
-include(CheckCXXSourceCompiles)
-
-# Some compilers (e.g. GCC < 9.1) do not link
-# std::filesystem/std::experimental::filesystem by default. If the sample
-# program can be linked, it means it is indeed linked by default. Otherwise, we
-# link it manually. https://en.cppreference.com/w/cpp/filesystem
-check_cxx_source_compiles(
-  "
-  #include <filesystem>
-
-  int main()
-  {
-    std::string path = \"/\";
-    return !std::filesystem::exists(path);
-  }
-  "
-  FILESYSTEM_LIBRARY_IS_LINKED
-)

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

@@ -179,10 +179,6 @@ function(build_lib)
       )
     endif()
 
-    if(NOT FILESYSTEM_LIBRARY_IS_LINKED)
-      list(APPEND BLIB_LIBRARIES_TO_LINK -lstdc++fs)
-    endif()
-
     # Enable examples as tests suites
     if(${ENABLE_EXAMPLES} AND ${ENABLE_TESTS})
       if(NOT ${XCODE})