automatically link to libstdc++-fs when we detect gcc8

No need for a FAQ in this case

CMakeLists.txt

@@ -38,6 +38,7 @@ include(GetGitRevisionDescription)
 include(ReplaceIdefixSource)
 include(AddIdefixSource)
 include(SetIdefixProperty)
+include(SetRequiredBuildSettingsForGCC8)
 
 #Idefix requires Cuda Lambdas (experimental)
 if(Kokkos_ENABLE_CUDA)
@@ -50,6 +51,9 @@ add_subdirectory(src/kokkos build/kokkos)
 # Add Idefix CXX Flags
 add_compile_options(${Idefix_CXX_FLAGS})
 
+# Add filesystem libraries for GCC8
+set_required_build_settings_for_GCC8()
+
 if(Idefix_WERROR)
   if(Kokkos_ENABLE_CUDA)
     add_compile_options(-Xcudafe --promote_warnings)

cmake/SetRequiredBuildSettingsForGCC8.cmake

@@ -0,0 +1,9 @@
+# The std::filesystem functions are part of the C++17 standard, but GCC8
+# does not store them in libstdc++ but in libstdc++-fs. This function
+# ensure that we link to this additional library when using GCC8
+# from https://discourse.cmake.org/t/correct-way-to-link-std-filesystem-with-gcc-8/4121
+function( set_required_build_settings_for_GCC8 )
+    # Always link with libstdc++fs.a when using GCC 8.
+    # Note: This command makes sure that this option comes pretty late on the cmdline.
+    link_libraries( "$<$<AND:$<CXX_COMPILER_ID:GNU>,$<VERSION_LESS:$<CXX_COMPILER_VERSION>,9.0>>:-lstdc++fs>" )
+endfunction()

doc/source/faq.rst

@@ -39,11 +39,6 @@ The compilation stops while compiling Kokkos with ``/usr/include/stdlib.h(58): e
 When using the Intel compiler on a Mac Intel, I get a linking error involving the ``SharedAllocationRecordIvvE18t_tracking_enabledE`` symbol.
   This is a known bug of the Intel Mac compiler with Kokkos. Apparently Intel has decided not to fix it. Check the issue on the `Kokkos git page <https://github.com/kokkos/kokkos/issues/1959>`_.
 
-I get an error at link that says there are undefined symbols fo std::filesystem
-  This is a known bug/limitation of gcc8 that does not include filesystem extensions in the standard library.
-  You should try to clear up CMakeCache.txt and explicitely add the required link library when calling cmake as in
-  ``LDFLAGS=-lstdc++fs cmake $IDEFIX_DIR ...```
-
 Execution
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