Replaced remaining _WIN32 symbols with WIN32

Stuff from work:

Removed reference to unused GL/glu.h header file, which is missing on
some Linux systems.

Fl_Gl_Window has a new method to allow you to get and set the context:

  void Fl_Gl_Window::context(void*, int destroy = 0)
  void* Fl_Gl_Window::context() const;

  Return or set a pointer to the GLContext that this window is
  using. This is a system-dependent structure, but it is portable to
  copy the context from one window to another. You can also set it to
  NULL, which will force FLTK to recreate the context the next time
  make_current() is called, this is useful for getting around bugs in
  OpenGL implementations.

  If destroy_flag is true the context will be destroyed by fltk when
  the window is destroyed, or when the mode() is changed, or the next
  time context(x) is called.

Some cleanup of Fl_Gl_Choice to move most of the system dependent
#ifdefs into Fl_Gl_Choice.cxx.


git-svn-id: file:///fltk/svn/fltk/branches/branch-1.0@1413 ea41ed52-d2ee-0310-a9c1-e6b18d33e121