Fix logic for configuring qt private headers

src/plugins/plugins.pro

@@ -39,15 +39,8 @@ contains(QT_CONFIG, declarative) {
 
     SUBDIRS += plugin_qmlprojectmanager
 
-    # Try to find location of Qt private headers (see README)
-
-    isEmpty(QT_PRIVATE_HEADERS) {
-        QT_PRIVATE_HEADERS = $$[QT_INSTALL_HEADERS]
-    } else {
-        INCLUDEPATH += QT_PRIVATE_HEADERS
-    }
-
-    exists($$QT_PRIVATE_HEADERS/QtDeclarative/private/qdeclarativecontext_p.h) {
+    include(private_headers.pri)
+    exists($${QT_PRIVATE_HEADERS}/QtDeclarative/private/qdeclarativecontext_p.h) {
         SUBDIRS += plugin_qmldesigner \
                    plugin_qmlinspector
     } else {

src/plugins/private_headers.pri

+# Try to find location of Qt private headers (see README)
+isEmpty(QT_PRIVATE_HEADERS) {
+    QT_PRIVATE_HEADERS = $$[QT_INSTALL_HEADERS]
+} else {
+    INCLUDEPATH += $$quote($${QT_PRIVATE_HEADERS}) \
+                   $$quote($${QT_PRIVATE_HEADERS}/QtDeclarative) \
+                   $$quote($${QT_PRIVATE_HEADERS}/QtCore) \
+                   $$quote($${QT_PRIVATE_HEADERS}/QtScript)
+}

src/plugins/qmldesigner/qmldesignerplugin.pro

@@ -2,6 +2,7 @@ TEMPLATE = lib
 TARGET = QmlDesigner
 
 include(../../qtcreatorplugin.pri)
+include(../private_headers.pri)
 include(qmldesigner_dependencies.pri)
 
 include(core/core.pri)

src/plugins/qmlinspector/qmlinspector.pro

@@ -4,6 +4,7 @@ TARGET = QmlInspector
 INCLUDEPATH += .
 DEPENDPATH += .
 
+include(../private_headers.pri)
 include(components/qmldebugger.pri)
 
 DEFINES += QMLINSPECTOR_LIBRARY