dropping some old platforms from the doc

[Bruno Haible]

Hi,

I'd like to remove the mentions of the following old platforms from
the gnulib documentation:

Platform         Comments
--------         --------
glibc < 2.2
Linux libc5
Mac OS X < 10.5
AIX < 5          end of support in 2004 (AIX 4.3.3 released in 1999)
HP-UX < 11       end of support in 2003 (HP-UX 10.30 released in 1997)
IRIX < 6.5       IRIX 6.5 released in 1998.
OSF/1            end of support in 2012 (OSF/1 5.1 released in 2000)
Solaris < 9      end of support in 2012 (Solaris 8 released in 2000)
Interix          last supported in Windows 8 (Interix 6.1 released in 2010)
BeOS             BeOS 5.1 released in 2001

The reason is that it starts to clutter up the documentation, when you're
looking for any more portability targets. Also, no one is testing these
platforms any more.

The corresponding code can still stay in gnulib, I don't mind. I'm
not religious about shorter or longer 'configure' execution times,
nor about the size of tarballs.

Bruno