autotools $(LN_S) fails if symlink/file already exists

[asavah]

In case of inplace upgrade libxcrypt 4.4.18 make install fails due to already existing symlink(s).

/home/asavah/kross/host/bin/make  install-exec-hook
make[4]: Entering directory '/home/asavah/kross/src/libxcrypt'
cd /home/asavah/kross/host/lib && \
        ln -s libcrypt.so libxcrypt.so
ln: failed to create symbolic link 'libxcrypt.so': File exists

All these and more will fail if file/symlink already exists.
https://github.com/besser82/libxcrypt/blob/develop/Makefile.am#L138-L140
https://github.com/besser82/libxcrypt/blob/develop/Makefile.am#L237-L239
https://github.com/besser82/libxcrypt/blob/develop/Makefile.am#L252-L254

Edit: quick and dirty workaround without butchering Makefile.am:
make install LN_S="ln -sf"