fix -system-sqlite configure option

Although configuration seemed to work and the configure summary
indicated that the system sqlite library was being used, qtsql
was actually still built from bundled sqlite sources. The problem
was that system-sqlite was not reaching the context of sqlite.pro.

Change-Id: I24676a19f0fc895111844cd0fe6c7e80f33574de
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/plugins/sqldrivers/sqlite/sqlite.pro

@@ -1,12 +1,14 @@
 TARGET = qsqlite
 
+QT_FOR_CONFIG += sql-private
+
 HEADERS += $$PWD/qsql_sqlite_p.h
 SOURCES += $$PWD/qsql_sqlite.cpp $$PWD/smain.cpp
 
-!system-sqlite {
-    include($$PWD/../../../3rdparty/sqlite.pri)
-} else {
+qtConfig(system-sqlite) {
     QMAKE_USE += sqlite
+} else {
+    include($$PWD/../../../3rdparty/sqlite.pri)
 }
 
 OTHER_FILES += sqlite.json

src/sql/configure.json

@@ -170,7 +170,7 @@
             "label": "  Using system provided SQLite",
             "autoDetect": false,
             "condition": "features.sql-sqlite && libs.sqlite3",
-            "output": [ "publicQtConfig" ]
+            "output": [ "privateFeature" ]
         },
         "sql-tds": {
             "label": "TDS (Sybase)",