make dist: Use `gnutar` automatically if it is found

`make dist` requires GNU tar for the `--transform` option.

`make dist` uses `tar` by default; on a GNU/Linux system, this is GNU
tar so it just works. On my OS X `tar` is BSD tar, but I can install GNU
tar as `gnutar` using macports.

Before this commit, I'd have to specify `gnutar` explicitly:
`make dist TAR=gnutar`.  With this commit, the Makefile will try to
figure out the right `tar` to use (if it finds `gnutar` it will use
that).

If `gnutar` isn't installed, the error message in the `make dist` recipe
still recommends specifying `TAR=gnutar`, as a hint that you should
install `gnutar`, or use `TAR=x` if you have installed GNU tar under a
different name.