build: Update FindFilesystem.cmake (#3905)

Vector-of-bool's `FindFilesystem.cmake` was updated at some point to include and explicit `CMAKE_CXX_STANDARD=17`. We've observed occasionally that on macOS, the find module would fail because it would default to C++14, where `std::filesystem` is not available (and I guess the experimental implementation has since been removed).

This PR updates the `FindFilesystem.cmake` to include this.

Fixes #3900

cmake/FindFilesystem.cmake

@@ -103,6 +103,8 @@ if(TARGET std::filesystem)
     return()
 endif()
 
+cmake_minimum_required(VERSION 3.10)
+
 include(CMakePushCheckState)
 include(CheckIncludeFileCXX)
 
@@ -124,6 +126,13 @@ cmake_push_check_state()
 
 set(CMAKE_REQUIRED_QUIET ${Filesystem_FIND_QUIETLY})
 
+# All of our tests required C++17 or later
+if(DEFINED CMAKE_CXX_STANDARD)
+    set(_prior_cmake_cxx_standard ${CMAKE_CXX_STANDARD})
+endif()
+
+set(CMAKE_CXX_STANDARD 17)
+
 # Normalize and check the component list we were given
 set(want_components ${Filesystem_FIND_COMPONENTS})
 if(Filesystem_FIND_COMPONENTS STREQUAL "")
@@ -282,6 +291,12 @@ set(Filesystem_FOUND
     FORCE
 )
 
+if(DEFINED _prior_cmake_cxx_standard)
+    set(CMAKE_CXX_STANDARD ${_prior_cmake_cxx_standard})
+else()
+    unset(CMAKE_CXX_STANDARD)
+endif()
+
 if(Filesystem_FIND_REQUIRED AND NOT Filesystem_FOUND)
     message(FATAL_ERROR "Cannot run simple program using std::filesystem")
 endif()