add to root-config option --platform which returns the ROOT name for the OS.

The existing option --arch returns the compiler/OS combo like "linuxicc"
where --platform returns "linux".

Also made win32gdk the default win32 version and moved win32 to win32old.
So to get the obsolete win32 version do: ./configure win32old.


git-svn-id: http://root.cern.ch/svn/root/trunk@9138 27541ba8-7e3a-0410-8455-c3a389f83636