[geary/mjog/webkit_plugin_warning-3_38] Components.WebView: Fix CI failure building with WebKitGTK >= 2.31

[Michael Gratton <mjog src gnome org>]

commit 5c79ca37794627aa79fa8ece69ea78f11cf46e93
Author: Michael Gratton <mike vee net>
Date:   Thu Jan 14 21:56:17 2021 +1100

    Components.WebView: Fix CI failure building with WebKitGTK >= 2.31
    
    The `webkit_settings_set_enable_plugins` method has been deprecated in
    2.31, and calling it emits a warning which trips up the test suite.
    
    Cherry picked from !624 commit 28921890

 src/client/components/client-web-view.vala | 2 ++
 src/client/meson.build                     | 4 ++++
 2 files changed, 6 insertions(+)
---
diff --git a/src/client/components/client-web-view.vala b/src/client/components/client-web-view.vala
index b65f21501..99a19105a 100644
--- a/src/client/components/client-web-view.vala
+++ b/src/client/components/client-web-view.vala
@@ -314,7 +314,9 @@ public abstract class ClientWebView : 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 088f4e47d..dd00554a2 100644
--- a/src/client/meson.build
+++ b/src/client/meson.build
@@ -199,6 +199,10 @@ if webkit_version[0].to_int() <= 2 and webkit_version[1].to_int() <= 24
   ]
 endif
 
+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,