speedo, w32: Add w32-wixlib target for MSI package

* Makefile.am (EXTRA_DIST): Add wixlib.wxs
* build-aux/speedo.mk (w32-wixlib): New target.
(w32-release): Build wixlib if WIXPREFIX is set.
(help): Add documentation.
* build-aux/speedo/w32/wixlib.wxs

This build a wixlib of the Windows binaries of GnuPG.
A wixlib is a module that can be linked into another
wix project to create an installer including this
module. Gpg4win uses the wixlib from GnuPG for
it's MSI Package.

To build the wixlib you need wine with wine-mono installed
and the wixtoolset.

When calling speedo set the variable WIXPREFIX to
the location containing the extracted toolset.

e.g.:

make -f build-aux/speedo.mk w32-wixlib WIXPREFIX=~/wix