[BUG] distclean gets confused by cp -sfR

From: Rob Landley
Date: Sun Feb 16 2020 - 06:55:17 EST

If you copy a dirty linux dir as a tree of symlinks and then "make distclean" in
the copy, most of the .o files get skipped.

$ cd linux; make defconfig; make -j $(nproc); cd ..
$ cp -sfR $PWD/linux boing
$ cd boing
$ make distclean
$ find . -name '*.o' | wc
892 892 20017

Reproduced in commit gb19e8c684703 on x86-64 (devuan ascii).