[x265] Building on Cygwin

[Steve Borho]

On 11/04, Phil Rosenberg wrote:
> 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.

I think the simplest answer is that on Windows we only support native
GCC (as MinGW with MSYS make and shell) or Visual Studio 2010 and above.

We do not test cygwin builds, if it works it is completely accidental.

-- 
Steve Borho