Since #8210 (59d063d), we've been passing -dbus-runtime when configuring
Qt.

qtbase-opensource-src-5.9.7 $ ./configure -h | grep -i dbus
  -no-dbus ............. Do not build the Qt D-Bus module
  -dbus-linked ......... Build Qt D-Bus and link to libdbus-1 [auto]
  -dbus-runtime ........ Build Qt D-Bus and dynamically load libdbus-1
[no]

This means we don't actually seem to be using the D-Bus we build in
depends. This was pointed out by theuni at the time, here and here, but
was never followed up. dongcarl also bought it up as part of #16150.