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>

src/Mod/Image/Gui/CMakeLists.txt

@@ -14,6 +14,12 @@ include_directories(
     ${XercesC_INCLUDE_DIRS}
 )
 
+if(MSVC)
+    include_directories(
+        ${CMAKE_SOURCE_DIR}/src/3rdParty/OpenGL/api
+    )
+endif(MSVC)
+
 set(ImageGui_LIBS
     Image
     FreeCADGui

src/Mod/Image/Gui/OpenGLImageBox.cpp

@@ -39,9 +39,13 @@
 # include <Windows.h>
 # include <GL/gl.h>
 # include <GL/glu.h>
+# if defined(_MSC_VER) && _MSC_VER >= 1910
+# include <GL/glext.h>
+# endif
 #else
 # include <GL/gl.h>
 # include <GL/glu.h>
+# include <GL/glext.h>
 #endif
 
 #include "OpenGLImageBox.h"