configure -libproxy: add a warning if not namespacing Qt

Task-number: QTBUG-46058
Change-Id: I62978b7fe9a82f9facb16ba56d0b14b792e3de61
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

src/network/configure.json

@@ -253,6 +253,19 @@
 library names through OPENSSL_LIBS.
 For example:
     OPENSSL_LIBS='-L/opt/ssl/lib -lssl -lcrypto' ./configure -openssl-linked"
+        },
+        {
+            "type": "warning",
+            "condition": "features.libproxy && input.qt_namespace == ''",
+            "message": "Some of libproxy's plugins may use incompatible Qt versions.
+
+    Some platforms and distributions ship libproxy with plugins, such
+    as config_kde4.so, that are linked against old versions of Qt; and
+    libproxy loads these plugins automatically when initialized. If Qt
+    is not in a namespace, that loading causes a crash. Even if the
+    systems on which you build and test have no such plugins, your
+    users' systems may have them. We therefore recommend that you
+    combine -libproxy with -qtnamespace when configuring Qt."
         }
     ],