Add a new CMake flag to disable deprecated functionality

Anything which is going to disappear can be #ifdef'd with this definition. This
is an additional hint if your 3rd party code is prepared for the next level of
QGIS.

For a start it's used to disable old labelling which causes deadlocks when used
with PyQt5-Python3.

CMakeLists.txt

@@ -668,6 +668,11 @@ IF (UNIX AND NOT APPLE)
   SET (QGIS_MANUAL_DIR  ${CMAKE_INSTALL_PREFIX}/${QGIS_MANUAL_SUBDIR})
 ENDIF (UNIX AND NOT APPLE)
 
+SET (DISABLE_DEPRECATED ${ENABLE_QT5} CACHE BOOL "If set to true, it will disable deprecated functionality to prepare for the next generation of QGIS")
+IF (DISABLE_DEPRECATED)
+  ADD_DEFINITIONS(-DQGIS_DISABLE_DEPRECATED)
+ENDIF (DISABLE_DEPRECATED)
+
 
 #############################################################
 # Python build dependency

ci/travis/linux/qt5/install.sh

@@ -35,6 +35,7 @@ cmake \
       -DWITH_SERVER=ON \
       -DENABLE_QT5=ON \
       -DENABLE_PYTHON3=ON \
+      -DDISABLE_DEPRECATED=ON \
       -DPORT_PLUGINS=ON \
       -DCXX_EXTRA_FLAGS="$CLANG_WARNINGS" \
       ..

src/core/qgsmaprenderercustompainterjob.cpp

@@ -293,9 +293,10 @@ void QgsMapRendererJob::drawLabeling( const QgsMapSettings& settings, QgsRenderC
   renderContext.setPainter( painter );
   renderContext.setLabelingEngine( labelingEngine );
 
+#if !defined(QGIS_DISABLE_DEPRECATED)
   // old labeling - to be removed at some point...
   drawOldLabeling( settings, renderContext );
-
+#endif
   drawNewLabeling( settings, renderContext, labelingEngine );
 
   if ( labelingEngine2 )