fix build failures with newer compiler versions

src/Gui/Quarter/QuarterWidget.cpp

@@ -167,6 +167,9 @@ public:
 #endif
         setFormat(surfaceFormat);
     }
+    ~CustomGLWidget()
+    {
+    }
     void initializeGL()
     {
         QOpenGLContext *context = QOpenGLContext::currentContext();
@@ -187,6 +190,13 @@ public:
     }
     void aboutToDestroyGLContext()
     {
+#if QT_VERSION >= 0x050900
+        // With Qt 5.9 a signal is emitted while the QuarterWidget is being destroyed.
+        // At this state its type is a QWidget, not a QuarterWidget any more.
+        QuarterWidget* qw = qobject_cast<QuarterWidget*>(parent());
+        if (!qw)
+            return;
+#endif
         QMetaObject::invokeMethod(parent(), "aboutToDestroyGLContext",
             Qt::DirectConnection,
             QGenericReturnArgument());

src/Gui/SoFCOffscreenRenderer.cpp

@@ -33,6 +33,12 @@
 # include <QImageWriter>
 #endif
 
+#if !defined(FC_OS_MACOSX)
+# include <GL/gl.h>
+# include <GL/glu.h>
+# include <GL/glext.h>
+#endif
+
 //gcc
 # include <iomanip>
 # include <ios>

src/Gui/View3DInventorViewer.cpp

@@ -85,6 +85,12 @@
 # include <QMimeData>
 #endif
 
+#if defined(FC_OS_MACOSX)
+# include <GL/gl.h>
+# include <GL/glu.h>
+# include <GL/glext.h>
+#endif
+
 #include <QVariantAnimation>
 
 #include <sstream>