Fix osgViewer `USE_GRAPHICSWINDOW` without a windowing system

There is no way for the user to know whether OSG was compiled with a
windowing system.

This means calling USE_GRAPHICSWINDOW() in client libraries was unsafe
in the case of the "None" windowing system, as it would fail to link.

Turns the call into a no-op in that case.

CMakeLists.txt

@@ -121,6 +121,9 @@ ELSE()
     SET(OSG_WINDOWING_SYSTEM "X11" CACHE STRING "Windowing system type for graphics window creation; options: X11 or None.")
 ENDIF()
 
+IF(OSG_WINDOWING_SYSTEM STREQUAL "None")
+    SET(OSG_WINDOWING_SYSTEM_NONE ON INTERNAL "No windowing system")
+ENDIF()
 
 SET(OPENSCENEGRAPH_VERSION ${OPENSCENEGRAPH_MAJOR_VERSION}.${OPENSCENEGRAPH_MINOR_VERSION}.${OPENSCENEGRAPH_PATCH_VERSION})
 

include/osgViewer/GraphicsWindow

@@ -283,7 +283,9 @@ struct GraphicsWindowFunctionProxy
     extern "C" void graphicswindow_##ext(void); \
     static osgViewer::GraphicsWindowFunctionProxy graphicswindowproxy_##ext(graphicswindow_##ext);
 
-#if defined(_WIN32)
+#if defined(OSG_WINDOWING_SYSTEM_NONE)
+    #define USE_GRAPHICS_WINDOW()
+#elif defined(_WIN32)
     #define USE_GRAPHICSWINDOW()  USE_GRAPICSWINDOW_IMPLEMENTATION(Win32)
 #elif defined(__APPLE__)
   #if defined(OSG_WINDOWING_SYSTEM_CARBON)

src/osg/Config.in

@@ -37,5 +37,6 @@
 #cmakedefine OSG_USE_DEPRECATED_API
 #cmakedefine OSG_ENVVAR_SUPPORTED
 #cmakedefine OSG_WINDOWING_SYSTEM_CARBON
+#cmakedefine OSG_WINDOWING_SYSTEM_NONE
 
 #endif