add ghc::filesystem as a submodule instead of including header directly in repo

.gitmodules

+[submodule "dune/copasi/common/ghc"]
+	path = ext/ghc
+	url = https://github.com/gulrak/filesystem

CMakeLists.txt

@@ -24,6 +24,10 @@ dune_project()
 
 dune_enable_all_packages()
 
+set(USE_FALLBACK_FILESYSTEM no CACHE BOOL "Use bundled std::filesystem alternative (enable this for macOS older than 10.15)")
+if(USE_FALLBACK_FILESYSTEM)
+  add_subdirectory(ext/ghc)
+endif()
 add_subdirectory(src)
 add_subdirectory(dune)
 add_subdirectory(lib)

dune/copasi/common/CMakeLists.txt

@@ -3,7 +3,6 @@ install(FILES bit_flags.hh
               enum.hh
               factory.hh
               filesystem.hh
-              ghc/filesystem.hpp
               muparser_data_handler.hh
               pdelab_expression_adapter.hh
               stepper.hh

dune/copasi/common/filesystem.hh

@@ -3,7 +3,7 @@
 
 #ifdef DUNE_COPASI_USE_FALLBACK_FILESYSTEM
 
-#include <dune/copasi/common/ghc/filesystem.hpp>
+#include <ghc/filesystem.hpp>
 namespace fs {
 using namespace ghc::filesystem;
 }

ghc @ d8abf146

+Subproject commit d8abf146a43ea12d28b09dfa71ece3bad4168185

lib/CMakeLists.txt

@@ -8,9 +8,9 @@ target_include_directories(dune_copasi_lib INTERFACE
   $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
 )
 
-set(USE_FALLBACK_FILESYSTEM no CACHE BOOL "Use bundled std::filesystem alternative (enable this for macOS older than 10.15)")
 if(USE_FALLBACK_FILESYSTEM)
   target_compile_definitions(dune_copasi_lib PUBLIC DUNE_COPASI_USE_FALLBACK_FILESYSTEM)
+  target_link_libraries(dune_copasi_lib PUBLIC ghc_filesystem)
 elseif(Filesystem_FOUND)
   target_link_libraries(dune_copasi_lib PUBLIC std::filesystem)
 endif()