fix DEPEND_CMD for out of tree builds

Originally committed as revision 12454 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -935,7 +935,7 @@ SLIBNAME_WITH_MAJOR='$(SLIBNAME).$(LIBMAJOR)'
 LIB_INSTALL_EXTRA_CMD='$(RANLIB) "$(LIBDIR)/$(LIBNAME)"'
 
 # gcc stupidly only outputs the basename of targets with -MM
-DEPEND_CMD='$(CC) -MM $(CFLAGS) $(filter-out %.h,$^) | sed "s,[0-9a-z._-]*: \([a-z0-9]*/\).*,\1&,"'
+DEPEND_CMD='$(CC) -MM $(CFLAGS) $(filter-out %.h,$^) | sed "s,[0-9a-z._-]*: \($(SRC_DIR)/\)*\([a-z0-9]*/\)[^/]* ,\2&,"'
 VHOOKDEPEND_CMD='$(CC) -MM $(VHOOKCFLAGS) $(filter-out %.h,$^) | sed "s,^\([a-z]\),vhook/\1,"'
 
 # find source path