QmlDesigner/QmlInspector: Document private headers dependency

Users also can now explicitly set the path to the private
header files by passing "QT_PRIVATE_HEADERS=$PATH" to qmake,
or force qmake to ignore the plugins by passing in an invalid
/empty path.

Reviewed-by: con

README

@@ -26,6 +26,31 @@ cd $BUILD_DIRECTORY
 qmake $SOURCE_DIRECTORY/qtcreator.pro
 make (or mingw32-make or nmake or jom, depending on your platform)
 
+QmlDesigner, QmlInspector require private headers
+-------------------------------------------------
+
+The QmlDesigner and QmlInspector plugins depend on "private" Qt headers,
+specifically from the QtDeclarative module. These private headers always end
+with an "_p.h", and Nokia does not make any promises to keep the files or API's
+binary or source compatible between releases. This means that when compiled,
+the two plugins have a dependency to the exact Qt version they were compiled
+with. Running Qt Creator with the plugins against updated Qt libraries (also for
+patch releases) might lead to link time failures, or even crashes.
+
+If you want to disable the plugins, you can pass "QT_PRIVATE_HEADERS=" to qmake:
+
+  qmake "QT_PRIVATE_HEADERS=" $SOURCE_DIRECTORY/qtcreator.pro
+
+Anyhow, the plugins will not be compiled when the private header files needed
+are not found. This might be the case when you are using a Qt version from your
+distribution, or when you installed your self-compiled Qt to a separate
+directory via 'make install'. You can fix this by either re-building your Qt
+with the "-developer-build" configure option, or pass the include directory in
+the source directory to qmake, e.g.
+
+  qmake "QT_PRIVATE_HEADERS=$$QT_SOURCE_TREE/include" $SOURCE_DIRECTORY/qtcreator.pro
+
+
 Third-party components
 ======================
 Qt Creator includes the following third-party components,

src/plugins/plugins.pro

@@ -37,13 +37,25 @@ SUBDIRS   = plugin_coreplugin \
 
 contains(QT_CONFIG, declarative) {
 
-    SUBDIRS += plugin_qmlprojectmanager \
-               plugin_qmlinspector
+    SUBDIRS += plugin_qmlprojectmanager
 
-    exists($$[QT_INSTALL_HEADERS]/QtDeclarative/private/qdeclarativecontext_p.h) {
-        SUBDIRS += plugin_qmldesigner
+    # Try to find location of Qt private headers (see README)
+
+    isEmpty(QT_PRIVATE_HEADERS) {
+        QT_PRIVATE_HEADERS = $$[QT_INSTALL_HEADERS]
+    } else {
+        INCLUDEPATH += QT_PRIVATE_HEADERS
+    }
+
+    exists($$QT_PRIVATE_HEADERS/QtDeclarative/private/qdeclarativecontext_p.h) {
+        SUBDIRS += plugin_qmldesigner \
+                   plugin_qmlinspector
     } else {
-        warning("QmlDesigner plugin disabled! It depends on private headers from QtDeclarative module, which seems not to be installed.")
+        warning()
+        warning("QmlDesigner and QmlInspector plugins have been disabled")
+        warning("The plugins depend on on private headers from QtDeclarative module.")
+        warning("To enable them, pass 'QT_PRIVATE_HEADERS=$QTDIR/include' to qmake, where $QTDIR is the source directory of qt.")
+        warning()
     }
 }