Add support for EXTRA_DIST_LINKS

Hi there,

I have created a patch for Automake to support a new Makefile variable
named EXTRA_DIST_LINKS. The purpose of the variable is that of creating
symlinks for the distributed archive.

The syntax is very simple: a space-separated list of tokens of the form
DEST:SOURCE – it is identical to that of AC_CONFIG_LINKS(dest:source ...)
in the configure.ac file.

For instance, adding the following lines to Makefile.am

EXTRA_DIST_LINKS = \
	my_link.txt:README \
	my_subdir/another_link.txt:NEWS

will create two links: my_link.txt (pointing to the README file in the same
directory) and my_subdir/another_link.txt (pointing to the NEWS file one
level above).

I had to hack also the tar.m4 file, to avoid dereferencing symlinks during
the creation of compressed archives. That should not be a problem, since
symlinks are anyway dereferenced one step before by make distdir, so with
this patch the only way to distribute a symlink is that of listing it
explicitly in EXTRA_DIST_LINKS. For the shell code I used this as base:
https://stackoverflow.com/a/12498485 (but I have made a few changes).

Please find the patch attached.

--madmurphy