meson: Handle features needed only by examples separately

Currently we incorrectly require gtk4 to build the fallbackswitch, livesync,
togglerecord plugins when the examples option is allowed.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1614>

meson.build

@@ -204,32 +204,6 @@ plugins = {
   },
 }
 
-if get_option('examples').allowed()
-  plugins += {
-    'fallbackswitch': {
-      'library': 'libgstfallbackswitch',
-      'examples': ['gtk-fallbackswitch'],
-      'features': ['gtk', 'gio', 'gst-plugin-gtk4'],
-    },
-    'livesync': {
-      'library': 'libgstlivesync',
-      'examples': ['gtk-livesync'],
-      'features': ['gtk', 'gio', 'gst-plugin-gtk4'],
-    },
-    'togglerecord': {
-      'library': 'libgsttogglerecord',
-      'examples': ['gtk-recording'],
-      'features': ['gtk', 'gio', 'gst-plugin-gtk4'],
-    },
-  }
-else
-  plugins += {
-    'fallbackswitch': { 'library': 'libgstfallbackswitch'},
-    'livesync': { 'library': 'libgstlivesync'},
-    'togglerecord': { 'library': 'libgsttogglerecord'},
-  }
-endif
-
 # Won't build on platforms where it bundles the sources because of:
 # https://github.com/qnighy/libwebp-sys2-rs/issues/12
 # the fix is:
@@ -329,6 +303,36 @@ if get_option('gtk4').allowed()
   endif
 endif
 
+examples_opt = get_option('examples')
+if examples_opt.allowed() and 'gtk4' in plugins
+  plugins += {
+    'fallbackswitch': {
+      'library': 'libgstfallbackswitch',
+      'examples_features': {
+        'gtk-fallbackswitch': ['gtk', 'gio', 'gst-plugin-gtk4'],
+      },
+    },
+    'livesync': {
+      'library': 'libgstlivesync',
+      'examples_features': {
+        'gtk-livesync': ['gtk', 'gio', 'gst-plugin-gtk4'],
+      }
+    },
+    'togglerecord': {
+      'library': 'libgsttogglerecord',
+      'examples_features': {
+        'gtk-recording': ['gtk', 'gio', 'gst-plugin-gtk4'],
+      }
+    },
+  }
+else
+  plugins += {
+    'fallbackswitch': { 'library': 'libgstfallbackswitch'},
+    'livesync': { 'library': 'libgstlivesync'},
+    'togglerecord': { 'library': 'libgsttogglerecord'},
+  }
+endif
+
 # Process plugins list
 
 default_library = get_option('default_library')
@@ -438,6 +442,26 @@ foreach plugin_name, details: plugins
     examples += example
   endforeach
 
+  plugin_examples_features = details.get('examples_features', {})
+  foreach example, examples_features: plugin_examples_features
+    example_deps_found = true
+    foreach feature: examples_features
+      if feature.startswith('gst-plugin') and not packages.contains(feature)
+        msg = f'@plugin_name@ example @example@ required feature @feature@ not found'
+        if plugin_opt.enabled() and examples_opt.enabled()
+          error(msg)
+        endif
+        message(msg + ', skipping')
+        example_deps_found = false
+        break
+      endif
+    endforeach
+    features += examples_features
+    if example_deps_found
+      examples += example
+    endif
+  endforeach
+
   packages += f'gst-plugin-@plugin_name@'
   features += plugin_features
   extra_features = run_command('dependencies.py', meson.current_source_dir(), plugin_name,