configure.ac: separate the "aac" and "mp4" decoder tests

MPD used to have a copy of the mp4ff library.  Since that has been
removed, AAC suport was disabled when there was no libmp4ff.  Separate
the libmp4ff test, and enable AAC support no matter if libmp4ff is
available.

src/decoder_list.c

@@ -50,9 +50,11 @@ static const struct decoder_plugin *const decoder_plugins[] = {
 	&audiofilePlugin,
 #endif
 #ifdef HAVE_FAAD
-	&mp4Plugin,
 	&aacPlugin,
 #endif
+#ifdef HAVE_MP4
+	&mp4Plugin,
+#endif
 #ifdef HAVE_MPCDEC
 	&mpcPlugin,
 #endif