Simplify GMX_X11 handling.

Now user request for GMX_X11 is always honored, or a fatal error given
if that is not possible instead of overriding the user-set option.
Use GMX_X11 consistently to turn on X11 compilation.
Reorganize code in view.cpp to have (nearly) all #ifdef'd code within a
single block for easier readability.  The main function is also now at
the end, making it easier to find by casual browsing.

Change-Id: Ic7f8908d117bacb6140f6e422c0e2c24bba85443