Executables are typically installed under bin/, sbin/ or libexec,
rather than lib/.  The problem was noticed when trying to use Jami
with a Guix relocatable pack, which only wraps executables found under
the above locations.  Since dring is a daemon and should not be run
directly by users under normal circumstances, the convention is to use
the libexec directory, according to the File Hierchy Standard 3.0.

Also tackled in this change is the substitution of the Autoconf
variables at installation time rather than configuration time, as per
the GNU Coding Standards.  The following solution is taken from
https://github.com/gentoo/eudev/pull/183/commits, which in turn is
adapted from the Autoconf manual (c.f.: info "(autoconf)Installation
Directory Variables").

* bin/Makefile.am (ringlibdir): Delete variable.
* bin/dbus/Makefile.am (do_subst): New variable.
(cx.ring.Ring.service): Derive from the cx.ring.Ring.service.in
template.
[HAVE_WIN32]: (ringlib_PROGRAMS): Rename to ...
(bin_PROGRAMS): ... this.
[HAVE_OSX, RING_DBUS]: Likewise.
* bin/dbus/cx.ring.Ring.service.in (Exec): Adust.
* globals.mk (ringlibdir): Delete variable.
* configure.ac [with_dbus] <AC_CONFIG_FILES>: Remove
bin/dbus/cx.ring.Ring.service.  Also remove the LIBDIR variable, no
longer used.  Fix indentation.

Thanks to Vladimir Stoiakin for providing feedback on this change.

Change-Id: I845bf2bd590a9457e70ca173027217b8f164d713