Deprecating platforms

[Chong Yidong]

I've just added the following note to etc/NEWS:

** The following platforms will be removed in a future Emacs version:
*** Libc version 5 on GNU/Linux.
*** Old FreeBSD and NetBSD systems based on the COFF executable
    format.
*** Solaris versions 2.6 and below.
*** Compilation using the NO_SHARED_LIBS flag.

The goal is to clean up the code by removing ifdefs catering to
platforms that no one is using.  If anyone knows of Emacs users based on
any of these old platforms, please let me know.