extplane-plugin: fix macOS build for Apple Silicon (X-Plane SDK 4.x)

[BhSimon]

Fix macOS build for Apple Silicon (X-Plane SDK 4.x)

Summary

This PR makes extplane-plugin/extplane-plugin.pro build a native arm64 mac.xpl,
verified loading and streaming datarefs in X‑Plane 12 on an M2 Mac.

All changes are in extplane-plugin/extplane-plugin.pro.

What was broken

1. CONFIG += x86 ppc in the macx {} block forced a 32‑bit Intel +
   PowerPC build. That fails on any modern macOS toolchain and makes an arm64
   build impossible. Removed — the plugin now builds for the architecture(s)
   of the Qt it links against (arm64 on Apple Silicon; universal if you link a
   universal Qt and add -arch flags).

2. XPLMCommandKeyStroke / XPLMCommandButtonPress / XPLMCommandButtonRelease
   (used in xplaneplugin.cpp) are gated behind XPLM_DEPRECATED in the
   X‑Plane SDK 4.x headers, so the plugin fails to compile with a current SDK:

   xplaneplugin.cpp:184:5: error: use of undeclared identifier 'XPLMCommandKeyStroke'
   

   The functions are still exported by XPLM.framework, so defining
   XPLM_DEPRECATED restores them. Harmless with SDK 3.x.

3. libmosquittopp was linked unconditionally via pkg-config, even though
   the MQTT sources are already guarded by CONFIG(mqtt). A default
   (non‑MQTT) build therefore failed unless mosquitto was installed. Moved the
   pkg-config link under the same CONFIG(mqtt) guard.

How to build on Apple Silicon

macOS 14+ can use Homebrew qt@5. On macOS 13 (Ventura) Homebrew has no
arm64 Qt5 bottle; conda‑forge works (its osx‑arm64 build targets macOS 11+):

# arm64 Qt5 (only needed if you don't have one)
mamba create -n xplane-qt5 -c conda-forge qt-main=5.15
export PATH="$CONDA_PREFIX/envs/xplane-qt5/bin:$PATH"

# X-Plane SDK 4.x next to the ExtPlane checkout (its Mac framework is arm64+x86_64)
#   ../XPlaneSDK  ->  SDK 4.x

cd ExtPlane && qmake && make          # builds extplane-plugin/extplane/mac.xpl (arm64)

Because the Qt5 comes from a non‑system prefix, the resulting .xpl references
Qt via @rpath; bundle libQt5Core, libQt5Network and their deps (ICU, z,
zstd) next to mac.xpl and rewrite install names to @loader_path for a
self‑contained plugin.

Verification

• Builds an arm64 mac.xpl against X‑Plane SDK 4.3.0 + Qt 5.15.
• minos 11.0, exports XPluginStart/Stop/Enable/Disable/ReceiveMessage.
• Loads in X‑Plane 12 on Apple Silicon: Loaded: .../ExtPlane/mac.xpl (org.vranki.extplaneplugin) and ExtPlane listening on TCP port 51000.
• Subscribing over TCP streams live datarefs (ud/uf/ui frames).
• In the same aircraft, x86_64‑only plugins failed with
  incompatible architecture (have 'x86_64', need 'arm64') — confirming
  X‑Plane ran native arm64 and this plugin matched.