When using contribs, don't set --with-a52-tree or --with-libmpeg2-tree

These should both have been installed, ergo no need to use them from their src directory. (Does the same go for cyberlink? who uses it, and where? )
Now configure.ac won't be forced into the --tree modes for these codecs (which still use BUILTINS ). The latter also shows us that we still have decprecated BUILTINS stuff to cleanup.

Fixes #1682

configure.ac

@@ -142,12 +142,6 @@ AC_ARG_WITH(contrib,
     if test -z $with_cyberlink_tree; then
        with_cyberlink_tree=${topdir}/extras/contrib/src/clinkcc
     fi
-    if test -z $with_libmpeg2_tree; then
-       with_libmpeg2_tree=${topdir}/extras/contrib/src/mpeg2dec
-    fi
-    if test -z $with_a52_tree; then
-       with_a52_tree=${topdir}/extras/contrib/src/a52dec
-    fi
 
     if test ".`uname -s`" = ".Darwin"; then
       export LD_LIBRARY_PATH=${topdir}/extras/contrib/lib:$LD_LIBRARY_PATH