Add AMR directories for out-of-tree builds.

based on a patch by Ramiro Polla, ramiro lisha.ufsc br

Originally committed as revision 7369 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2029,6 +2029,9 @@ if test "$source_path_used" = "yes" ; then
          libavcodec/mlib \
          libavcodec/ppc \
          libavcodec/liba52 \
+         libavcodec/amr \
+         libavcodec/amr_float \
+         libavcodec/amrwb_float \
          libpostproc \
          libavutil \
          libswscale \