Add (guix lzlib).

* guix/lzlib.scm, tests/lzlib.scm: New files.
* Makefile.am (MODULES): Add guix/lzlib.scm.
(SCM_TESTS): Add tests/lzlib.scm.
* m4/guix.m4 (GUIX_LIBLZ_LIBDIR): New macro.
* configure.ac (LIBLZ_LIBDIR): Use it.  Define and substitute
'LIBLZ'.
* guix/config.scm.in (%liblz): New variable.
* guix/self.scm (make-config.scm): Add TODO comment.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>