buildbot: take D-Bus from the system on Linux

The Linux jobs keep dying in vcpkg install, not on anything we build: the
dbus port fetches its tarball from gitlab.freedesktop.org, and that host has
answered 504 to all three of vcpkg's retries twice in a row now - the
aarch64 job in one pipeline, the x86_64 job in the next. Rerunning is not a
fix, so stop building the port at all.

An empty overlay port, the same trick this tree already uses for gtk3 and
cairo, is enough. Nothing in Cemu talks to D-Bus directly; the only consumer
is sdl2[dbus] (a default feature of the port, so dropping our own dependency
entry would not have helped), which dlopen()s libdbus-1 at run time and
needs no more than the headers to build. libdbus-1-dev is already installed
in before_script and covers those.

.gitlab-ci.yml

@@ -72,10 +72,10 @@ stages:
     # Repeated from .libretro-linux-cmake, whose before_script this replaces.
     - export NUMPROC=$(($(nproc)/5))
     - if builtin type -P ccache &> /dev/null; then ccache -V &> /dev/null; fi
-    # glm comes from the system: dependencies/vcpkg_overlay_ports_linux/glm is
-    # an empty package on purpose, so libglm-dev has to be installed. The rest
-    # is what the vcpkg ports (SDL2, dbus, libusb) and the core itself
-    # (GL/EGL/X11) need to be built.
+    # libdbus-1-dev is not optional here: dependencies/vcpkg_overlay_ports_linux/dbus
+    # is an empty package on purpose (see the portfile), so sdl2[dbus] has only
+    # the system headers to build against. The rest is what the other vcpkg
+    # ports (SDL2, libusb) and the core itself (GL/EGL/X11) need.
     - apt-get update -qq
     - apt-get install -y --no-install-recommends
       nasm autoconf automake libtool pkg-config python3

dependencies/vcpkg_overlay_ports_linux/dbus/portfile.cmake

@@ -0,0 +1,12 @@
+# Same deal as the gtk3 and cairo overlays: take D-Bus from the system rather
+# than building it here.
+#
+# The upstream port fetches its tarball from gitlab.freedesktop.org, which has
+# been answering 504 for it often enough to fail whole pipelines (all three
+# vcpkg retries in a row, twice running). Nothing in Cemu talks to D-Bus
+# directly - the only consumer is sdl2[dbus], which dlopen()s libdbus-1 at run
+# time and needs no more than the headers to build, so the distro's
+# libdbus-1-dev covers it. Where that is missing, SDL2 simply builds without
+# D-Bus support, which costs a libretro core nothing: the frontend owns the
+# window, the screen saver and the input method.
+set(VCPKG_POLICY_EMPTY_PACKAGE enabled)

dependencies/vcpkg_overlay_ports_linux/dbus/vcpkg.json

@@ -0,0 +1,8 @@
+{
+  "name": "dbus",
+  "version": "1.16.2",
+  "port-version": 1,
+  "description": "D-Bus specification and reference implementation, taken from the system instead of built from source.",
+  "homepage": "https://www.freedesktop.org/wiki/Software/dbus/",
+  "license": null
+}