cmake: Add CheckStdFilesystem module

CMakeLists.txt

@@ -101,6 +101,9 @@ include(cmake/introspection.cmake)
 include(cmake/crc32c.cmake)
 include(cmake/leveldb.cmake)
 
+include(CheckStdFilesystem)
+check_std_filesystem()
+
 add_subdirectory(src)
 
 message("\n")

cmake/module/CheckStdFilesystem.cmake

@@ -0,0 +1,36 @@
+# Copyright (c) 2023 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+# GCC 8.x (libstdc++) requires -lstdc++fs
+# Clang 8.x (libc++) requires -lc++fs
+
+function(check_std_filesystem)
+  set(source "
+    #include <filesystem>
+
+    int main()
+    {
+      (void)std::filesystem::current_path().root_name();
+    }
+  ")
+
+  include(CheckCXXSourceCompiles)
+  check_cxx_source_links("${source}" STD_FILESYSTEM_NO_EXTRA_LIBS_NEEDED)
+  if(STD_FILESYSTEM_NO_EXTRA_LIBS_NEEDED)
+    return()
+  endif()
+
+  add_library(std_filesystem INTERFACE)
+  check_cxx_source_links_with_libs(stdc++fs "${source}" STD_FILESYSTEM_NEEDS_LINK_TO_LIBSTDCXXFS)
+  if(STD_FILESYSTEM_NEEDS_LINK_TO_LIBSTDCXXFS)
+    target_link_libraries(std_filesystem INTERFACE stdc++fs)
+    return()
+  endif()
+  check_cxx_source_links_with_libs(c++fs "${source}" STD_FILESYSTEM_NEEDS_LINK_TO_LIBCXXFS)
+  if(STD_FILESYSTEM_NEEDS_LINK_TO_LIBCXXFS)
+    target_link_libraries(std_filesystem INTERFACE c++fs)
+    return()
+  endif()
+  message(FATAL_ERROR "Cannot figure out how to use std::filesystem.")
+endfunction()