Whither windows-msvc

[John Ericson]

Note that `windows-msvc` is not supported by GCC. It is only supported 
by Microsoft's own proprietary tools, and LLVM. Thus, the only free 
software implementation of this target calls it `windows-msvc`.

`winnt` I do see in config.log, but I have never seen used in the wild. 
I think it is just detritus from some obscure Alpha CPU thing. It is (a) 
untested, and (b) almost removed in 
554e3feb158dbde3193c05fddec422c3c93531cd.

See 
https://lists.gnu.org/archive/html/config-patches/2017-09/msg00000.html 
for first message in the thread that led to that commit; In Ben's own 
words (in a reply to that original message):

> Thanks. I'm not normally inclined to remove triplets (because I find
> it a useful way to preserve the history!) but in this case, it's too
> confusing for all of these defunct attempts to do Unix on Windows.

This `winnt`is but more detritus from such defunct attempts. I would 
deprecate it so it no longer confuses people. (Deprecation I think is 
better than sudden removal.)

John