Installation notes to build Unison on Windows systems

We provide two options for building Unison on MS Windows.
Both options require the Cygwin layer to be able to use a few GNU
tools as well as the OCaml distribution version 3.04.  The options
differ in the C compiler employed: MS Visual C++ (MSVC) vs Cygwin GNU C.

Tradeoff? 
. Only the MSVC option can produce statically linked Unison executable.  
. The Cygwin GNU C option requires only free software.

The files "INSTALL.win32-msvc" and "INSTALL.win32-cygwin-gnuc" describe
the building procedures for the respective options.

