From Norman Vine, fixes for Mingw

include/osg/ConvexPlanarOccluder

@@ -54,7 +54,7 @@ class SG_EXPORT ConvexPlanarOccluder : public Object
 
     protected:
 
-        ~ConvexPlanarOccluder() {}
+        ~ConvexPlanarOccluder(); // {}
 
         ConvexPlanarPolygon _occluder;
         HoleList _holeList;

include/osg/GLExtensions

@@ -19,7 +19,9 @@
 
 #if defined(WIN32)
     #define WIN32_LEAN_AND_MEAN
-    #define NOMINMAX
+    #ifndef NOMINMAX
+        #define NOMINMAX
+    #endif // NOMINMAX
     #include <windows.h>
 #elif defined(__APPLE__)
     #include <mach-o/dyld.h>

include/osg/Shape

@@ -82,7 +82,7 @@ class SG_EXPORT Shape : public Object
 
     protected:
     
-    	virtual ~Shape() {}
+    	virtual ~Shape();
 };
 
 // forward declartions of Shape types.
@@ -571,7 +571,7 @@ class SG_EXPORT HeightField : public Shape
 
     protected:
     
-    	~HeightField() {}
+    	virtual ~HeightField();
 
     	unsigned int    _columns,_rows;