[geary/mjog/webkit_plugin_warning] Components.WebView: Fix CI failure building with WebKitGTK >= 2.21

[Michael Gratton <mjog src gnome org>]

commit fea6395c4fe780febf62d3efc4bef1f04c27ba85
Author: Michael Gratton <mike vee net>
Date:   Thu Jan 14 21:56:17 2021 +1100

    Components.WebView: Fix CI failure building with WebKitGTK >= 2.21
    
    The `webkit_settings_set_enable_plugins` method has been deprecated, but
    calling it emits a warning which trips up the test suite.

 src/client/components/components-web-view.vala | 2 ++
 src/client/meson.build                         | 4 ++++
 2 files changed, 6 insertions(+)
---
diff --git a/src/client/components/components-web-view.vala b/src/client/components/components-web-view.vala
index 904c5358a..361b08dc3 100644
--- a/src/client/components/components-web-view.vala
+++ b/src/client/components/components-web-view.vala
@@ -325,7 +325,9 @@ public abstract class Components.WebView : WebKit.WebView, Geary.BaseInterface {
         setts.enable_media_stream = false;
         setts.enable_offline_web_application_cache = false;
         setts.enable_page_cache = false;
+#if WEBKIT_PLUGINS_SUPPORTED
         setts.enable_plugins = false;
+#endif
         setts.hardware_acceleration_policy =
             WebKit.HardwareAccelerationPolicy.NEVER;
         setts.javascript_can_access_clipboard = true;
diff --git a/src/client/meson.build b/src/client/meson.build
index 54682c823..aeea36d01 100644
--- a/src/client/meson.build
+++ b/src/client/meson.build
@@ -192,6 +192,10 @@ client_vala_args += [
   )
 ]
 
+if webkit2gtk.version().version_compare('<2.31')
+  client_vala_args += [ '--define=WEBKIT_PLUGINS_SUPPORTED' ]
+endif
+
 # Main client application library
 client_lib = shared_library(
   client_package,