chore: use 3.20's cmake_path if available

CMakeLists.txt

@@ -282,11 +282,17 @@ if(PYBIND11_INSTALL)
       set(prefix_for_pc_file "${CMAKE_INSTALL_PREFIX}")
     else()
       set(pc_datarootdir "${CMAKE_INSTALL_DATAROOTDIR}")
-      set(prefix_for_pc_file "\${pcfiledir}/..")
-      while(pc_datarootdir)
-        get_filename_component(pc_datarootdir "${pc_datarootdir}" DIRECTORY)
-        string(APPEND prefix_for_pc_file "/..")
-      endwhile()
+      if(CMAKE_VERSION VERSION_LESS 3.20)
+        set(prefix_for_pc_file "\${pcfiledir}/..")
+        while(pc_datarootdir)
+          get_filename_component(pc_datarootdir "${pc_datarootdir}" DIRECTORY)
+          string(APPEND prefix_for_pc_file "/..")
+        endwhile()
+      else()
+        cmake_path(RELATIVE_PATH pcfiledir
+                   BASE_DIRECTORY CMAKE_INSTALL_DATAROOTDIR
+                   OUTPUT_VARIABLE prefix_for_pc_file)
+      endif()
     endif()
   endif()
   join_paths(includedir_for_pc_file "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}")