core: fix build with glib < 2.34

Using g_clear_pointer() didn't trigger G_UNAVAILABLE warnings because
it's a macro. Fix the build on older glibs by just pulling in the
definition.

include/nm-glib-compat.h

@@ -36,6 +36,25 @@ g_type_ensure (GType type)
   if (G_UNLIKELY (type == (GType)-1))
     g_error ("can't happen");
 }
+
+#define g_clear_pointer(pp, destroy)	  \
+	G_STMT_START { \
+		G_STATIC_ASSERT (sizeof *(pp) == sizeof (gpointer)); \
+		/* Only one access, please */ \
+		gpointer *_pp = (gpointer *) (pp); \
+		gpointer _p; \
+		/* This assignment is needed to avoid a gcc warning */ \
+		GDestroyNotify _destroy = (GDestroyNotify) (destroy); \
+	  \
+		(void) (0 ? (gpointer) *(pp) : 0); \
+		do \
+			_p = g_atomic_pointer_get (_pp); \
+		while G_UNLIKELY (!g_atomic_pointer_compare_and_exchange (_pp, _p, NULL)); \
+	  \
+		if (_p) \
+			_destroy (_p); \
+	} G_STMT_END
+
 #endif
 
 #endif  /* NM_GLIB_COMPAT_H */

src/config/nm-config.c

@@ -26,6 +26,7 @@
 #include "nm-config.h"
 #include "nm-logging.h"
 #include "nm-utils.h"
+#include "nm-glib-compat.h"
 
 #include <gio/gio.h>
 #include <glib/gi18n.h>