[vtkusers] VTK build instructions cleanup

[David Doria]

Hi all,

I have reorganized the build instructions a little bit. We should
strive to have good instructions so that external tutorials (of which
there have been way too many written over the years) are not necessary
for people to figure out how to build VTK.

The URL is now very simple:

http://www.itk.org/Wiki/VTK/Building

from there, the user chooses his operating system (Linux, OSX, or
Windows) and is directed to the appropriate page:

http://www.itk.org/Wiki/VTK/Building/Linux
http://www.itk.org/Wiki/VTK/Building/MacOSX
http://www.itk.org/Wiki/VTK/Building/Windows

If someone (or many people!) who uses each of these operating systems
could review the instructions and make sure they are up to date, that
would be great.

Thanks,

David