configure: keep the replacement libsystemd opt-in

Building it by default installs our libsystemd.so.0 in $libdir.  Where
the real one is already present, and contrib/debian/build.sh configures
--prefix=/usr --exec-prefix= on a Debian host, ours outranks it in the
loader cache, and every program linking libsystemd loses
sd_journal_stream_fd and the LIBSYSTEMD_209 symbol versions.  The test
sysroot ran into exactly that, dbus-daemon exited 127 on every restart.

Restore the default to no.  sulogin and watchdogd stay on, they only add
binaries.  distcheck asks for the library so it stays covered.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/ChangeLog.md

@@ -69,12 +69,10 @@ All relevant changes are documented in this file.
   `tmpfiles.d/dbus.conf`, so both can be overridden from `/etc` like any other
   system file.  Those directories are no longer chowned to `messagebus`,
   matching how Finit ships directories for other daemons
-- The bundled `sulogin` and `watchdogd`, and the replacement `libsystemd`,
-  are now built and installed by default, like keventd and the D-Bus
-  support.  `--without-sulogin`, `--without-watchdog`, and
-  `--without-libsystemd` opt out.  Note, Finit starts `watchdogd` by
-  itself when the WDT device node exists, and `libsystemd.so` is
-  installed to `$libdir`, where a system with systemd already has one
+- The bundled `sulogin` and `watchdogd` are now built and installed by
+  default, like keventd and the D-Bus support.  `--without-sulogin` and
+  `--without-watchdog` opt out.  Note, Finit starts `watchdogd` by itself
+  when the WDT device node exists
 - The `tty` block takes `passenv`, which the line-based format has had since
   v4.4 (issue #286) and the block format was missing
 - New `provides` setting for run/task/service/sysv blocks, naming conditions