Add NEWS entries for #180

Signed-off-by: Simon McVittie <smcv@collabora.com>

NEWS

 dbus 1.15.4 (UNRELEASED)
 ========================
 
+Dependencies:
+
+• Building with CMake now requires CMake ≥ 3.9.
+
+Build-time configuration changes:
+
+• On Unix platforms, a path in the runtime state directory (often /run)
+  is now used for the well-known system bus socket by default. OS
+  distributors should check that the path used is equivalent to the
+  interoperable path /var/run/dbus/system_bus_socket, especially if
+  running on an OS where /var/run is not guaranteed to be a symbolic
+  link to /run.
+  (dbus#180; Issam E. Maghni, Simon McVittie)
+  · With Autotools, this is controlled by --runstatedir, which defaults
+    to ${localstatedir}/run but is often set to /run by OS distributors.
+    The path to the system bus socket can be overridden with the
+    --with-system-socket option if required.
+  · With CMake, this is controlled by the RUNSTATEDIR option, which has
+    behaviour similar to Autotools. There is no separate option for the
+    path to the system bus socket.
+  · With Meson, this is controlled by the runtime_dir option, which
+    defaults to /run if the installation prefix is set to /usr, or has
+    behaviour similar to Autotools otherwise. The path to the system bus
+    socket can be overridden with the system_socket option if required.
+
 New API:
 
 • Add dbus_connection_set_builtin_filters_enabled(), intended to be called