Escape \1 and \2 in sed expressions written to config.mak to avoid mangling.

Originally committed as revision 12462 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -935,8 +935,8 @@ 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._-]*: \($(SRC_DIR)/\)*\([a-z0-9]*/\)[^/]* ,\2&,"'
-VHOOK_DEPEND_CMD='$(CC) -MM $(VHOOKCFLAGS) $(filter-out %.h,$^) | sed "s,^\([a-z]\),vhook/\1,"'
+DEPEND_CMD='$(CC) -MM $(CFLAGS) $(filter-out %.h,$^) | sed "s,[0-9a-z._-]*: \($(SRC_DIR)/\)*\([a-z0-9]*/\)[^/]* ,\\2&,"'
+VHOOK_DEPEND_CMD='$(CC) -MM $(VHOOKCFLAGS) $(filter-out %.h,$^) | sed "s,^\([a-z]\),vhook/\\1,"'
 
 # find source path
 source_path="`dirname \"$0\"`"