OpenGL2: Fix undeclared "glXGetProcAddressARB" when building on Centos5

This commit fixes the build error reported below when building VTK
on Centos 5.11 where mesa 6.5.1 is used.

Not that the compiler used is a newer version of g++ with c++11 support.
See [1]


  VTK/Rendering/OpenGL2/vtkXOpenGLRenderWindow.cxx:581:76: error: ‘glXGetProcAddressARB’ was not declared in this scope
       glXGetProcAddressARB( (const GLubyte *) "glXCreateContextAttribsARB" );

[1] https://access.redhat.com/documentation/en-US/Red_Hat_Developer_Toolset/2/html/2.0_Release_Notes/DTS2.0_Release.html

Rendering/OpenGL2/vtkXOpenGLRenderWindow.cxx

@@ -21,6 +21,9 @@
 // provided by the system.
 //#define GLX_GLXEXT_LEGACY
 
+// Ensure older version of glx.h define glXGetProcAddressARB
+#define GLX_GLXEXT_PROTOTYPES
+
 // New Workaround:
 // The GLX_GLXEXT_LEGACY definition was added to work around system glxext.h
 // files that used the GLintptr and GLsizeiptr types, but did not define them.