bug#19989: 25.0.50; Build instructions on Windows

[Eli Zaretskii]

> Date: Fri, 6 Mar 2015 01:59:52 -0800
> From: Ilya Zakharevich <ilya@math.berkeley.edu>
> Cc: Eli Zaretskii <eliz@gnu.org>, 19989@debbugs.gnu.org
> 
>   A) apparently, the download and build instructions changes not so
>      long ago, so what google finds are obsolete irrelevant
>      instructions.

The build instructions changed in May 2013, almost 2 years ago.

> The warning which nt/configure.bat gives is not specific enough (at
> least when shown on background of all the [wrong] info found by googling):

If no one complains about something major for the MinGW/MSYS build
when Emacs 24.5 is released, nt/configure.bat will be deleted from the
repository, so it won't be able to confuse anyone anymore.