bug#22959: Emacs on Windows depends on libwinpthreads

[Phillip Lord]

Currently, building Emacs under msys2/ming-w64 produces a binary that
depends on libwinpthread.dll. The practical upshot of this is that after
building and installing Emacs according to the instructions, Emacs
cannot be launched from the Windows explorer -- it can be run from msys2
which has the path set up correctly. A binary release will, therefore, fail.

Using the dependency walker shows the dependency is directly from Emacs,
and it appears to have come from a change in ming-w64, as reported here.

https://sourceforge.net/p/mingw-w64/mailman/message/31213279/

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=748353

This affects emacs-25, master and, indeed, emacs-24 built using the
current tool chain.