Don't redefine G_DEFINE_AUTOPTR_CLEANUP_FUNC when available in polkit.

Taken from upstream:

  https://gitlab.gnome.org/GNOME/gnome-shell/-/commit/1d0a08b5e25fea7b0e792ec9798e68a7c5606a75

diff --git a/config.h.meson b/config.h.meson
index b93fda8727..ff355d3062 100644
--- a/config.h.meson
+++ b/config.h.meson
@@ -33,3 +33,6 @@
 
 /* Define if fdwalk is available in libc */
 #mesondefine HAVE_FDWALK
+
+/* Define if polkit defines autocleanup functions */
+#mesondefine HAVE_POLKIT_AUTOCLEANUP
diff --git a/meson.build b/meson.build
index 42ec01c566..778a34c6ef 100644
--- a/meson.build
+++ b/meson.build
@@ -169,6 +169,13 @@ cdata.set('HAVE_FDWALK',
           cc.has_function('fdwalk')
 )
 
+polkit_has_autocleanup = cc.compiles(
+  '#define POLKIT_AGENT_I_KNOW_API_IS_SUBJECT_TO_CHANGE
+  #include <polkitagent/polkitagent.h>
+  void main(void) { g_autoptr(PolkitAgentListener) agent = NULL; }',
+  dependencies: polkit_dep)
+cdata.set('HAVE_POLKIT_AUTOCLEANUP', polkit_has_autocleanup)
+
 buildtype = get_option('buildtype')
 if buildtype != 'plain'
   all_warnings = [
diff --git a/src/shell-polkit-authentication-agent.h b/src/shell-polkit-authentication-agent.h
index 55b46af110..4f14749563 100644
--- a/src/shell-polkit-authentication-agent.h
+++ b/src/shell-polkit-authentication-agent.h
@@ -14,8 +14,10 @@
 
 G_BEGIN_DECLS
 
+#ifndef HAVE_POLKIT_AUTOCLEANUP
 /* Polkit doesn't have g_autoptr support, thus we have to manually set the autoptr function here */
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (PolkitAgentListener, g_object_unref)
+#endif
 
 #define SHELL_TYPE_POLKIT_AUTHENTICATION_AGENT (shell_polkit_authentication_agent_get_type())