[x265] Building on Cygwin

[Phil Rosenberg]

Oh, and you might also want to note the following warning that is
generated when running CMake on Cygwin

CMake Warning at
/usr/share/cmake-3.3.1/Modules/Platform/CYGWIN.cmake:15 (message):
CMake no longer defines WIN32 on Cygwin!
(1) If you are just trying to build this project, ignore this warning or
quiet it by setting CMAKE_LEGACY_CYGWIN_WIN32=0 in your environment or in
the CMake cache. If later configuration or build errors occur then this
project may have been written under the assumption that Cygwin is WIN32.
In that case, set CMAKE_LEGACY_CYGWIN_WIN32=1 instead.
(2) If you are developing this project, add the line
set(CMAKE_LEGACY_CYGWIN_WIN32 0) # Remove when CMake >= 2.8.4 is required
at the top of your top-level CMakeLists.txt file or set the minimum
required version of CMake to 2.8.4 or higher. Then teach your project to
build on Cygwin without WIN32.

Phil