Add qtwebengine backend to qbs help plugin build rules

Change-Id: I924637581741dc68d94270cc680df20499778b03
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/help/help.qbs

@@ -5,6 +5,7 @@ QtcPlugin {
 
     Depends { name: "Qt"; submodules: ["help", "network", "sql"]; }
     Depends { name: "Qt.printsupport" }
+    Depends { name: "Qt.webenginewidgets"; required: false }
 
     Depends { name: "Aggregation" }
     Depends { name: "Utils" }
@@ -16,6 +17,8 @@ QtcPlugin {
 
     cpp.defines: {
         var defines = base.concat(["QT_CLUCENE_SUPPORT"]);
+        if (Qt.webenginewidgets.present)
+            defines.push("QTC_WEBENGINE_HELPVIEWER");
         return defines;
     }
 
@@ -50,6 +53,14 @@ QtcPlugin {
         ]
     }
 
+    Group {
+        name: "WebEngine Sources"
+        condition: Qt.webenginewidgets.present
+        files: [
+            "webenginehelpviewer.cpp", "webenginehelpviewer.h"
+        ]
+    }
+
     Group {
         id: sharedSources
         name: "Shared Sources"