Bring back ghc::filesystem for broke AF macos

macOS doesn't provide `<filesystem>` support when targetting anything
earlier than 10.15.

.gitmodules

@@ -7,6 +7,9 @@
 [submodule "external/cxxopts"]
 	path = external/cxxopts
 	url = https://github.com/jarro2783/cxxopts.git
+[submodule "external/ghc-filesystem"]
+	path = external/ghc-filesystem
+	url = https://github.com/gulrak/filesystem.git
 [submodule "test/Catch2"]
 	path = test/Catch2
 	url = https://github.com/catchorg/Catch2

CMakeLists.txt

@@ -1,6 +1,7 @@
 cmake_minimum_required(VERSION 3.10)  # bionic's cmake version
 
-set(CMAKE_OSX_DEPLOYMENT_TARGET 10.13)  # Has to be set before `project()`, and ignored on non-macos
+# Has to be set before `project()`, and ignored on non-macos:
+set(CMAKE_OSX_DEPLOYMENT_TARGET 10.13 CACHE STRING "macOS deployment target (Apple clang only)")
 
 find_program(CCACHE_PROGRAM ccache)
 if(CCACHE_PROGRAM)
@@ -261,6 +262,7 @@ if(SUBMODULE_CHECK)
     check_submodule(external/nlohmann)
     check_submodule(external/googletest)
     check_submodule(external/cxxopts)
+    check_submodule(external/ghc-filesystem)
     check_submodule(external/date)
     check_submodule(external/pybind11)
     check_submodule(external/libuv)

cmake/check_for_std_filesystem.cmake

@@ -15,6 +15,13 @@ int main() {
 }
 ]])
 
+if(CMAKE_CXX_COMPILER STREQUAL "AppleClang" AND CMAKE_OSX_DEPLOYMENT_TARGET)
+  # It seems that check_cxx_source_compiles doesn't respect the CMAKE_OSX_DEPLOYMENT_TARGET, so this
+  # check would pass on Catalina (10.15) and then later compilation would fail because you aren't
+  # allowed to use <filesystem> when the deployment target is anything before 10.15.
+  set(CMAKE_REQUIRED_FLAGS -mmacosx-version-min=${CMAKE_OSX_DEPLOYMENT_TARGET})
+endif()
+
 check_cxx_source_compiles("${filesystem_code}" filesystem_compiled)
 if(filesystem_compiled)
   message(STATUS "No extra link flag needed for std::filesystem")
@@ -35,5 +42,9 @@ unset(CMAKE_REQUIRED_LIBRARIES)
 if(filesystem_is_good EQUAL 1)
   message(STATUS "we have std::filesystem")
 else()
-  message(FATAL_ERROR "we don't have std::filesystem")
+  # Probably broken AF macos
+  message(STATUS "std::filesystem is not available, apparently this compiler isn't C++17 compliant; falling back to ghc::filesystem")
+  add_subdirectory(external/ghc-filesystem)
+  target_link_libraries(filesystem INTERFACE ghc_filesystem)
+  target_compile_definitions(filesystem INTERFACE USE_GHC_FILESYSTEM)
 endif()

external/ghc-filesystem

@@ -0,0 +1 @@
+Subproject commit e63a58c5bac94a3a75a7083f87bb092531407a92

llarp/config/config.cpp

@@ -16,7 +16,6 @@
 #include <fstream>
 #include <ios>
 #include <iostream>
-#include <filesystem>
 
 namespace llarp
 {

llarp/util/fs.hpp

@@ -9,8 +9,13 @@
 #define PATH_SEP "/"
 #endif
 
+#ifdef USE_GHC_FILESYSTEM
+#include <ghc/filesystem.hpp>
+namespace fs = ghc::filesystem;
+#else
 #include <filesystem>
 namespace fs = std::filesystem;
+#endif
 
 #ifndef _MSC_VER
 #include <dirent.h>