[CMake] cmake for cygwin

[Bill Hoffman]

On 10/24/2010 11:52 PM, Yaakov (Cygwin/X) wrote:
> On Fri, 2010-10-22 at 13:12 +0000, Rolf Eike Beer wrote:
>> So this is only different from what other build tools or whatever do. But
>> it is well known behaviour, it is documented, and it can't be changed for
>> backward compatibility anyway.
>
> The only platform which this affects is Cygwin, we (the Cygwin package
> maintainers) regard this as a bug, and therefore are not concerned with
> backward compatibility.
>
> Let me make this clear: with WIN32 defined, CMake is practically
> *useless* to us on Cygwin, as it requires patching almost every
> CMake-based package to build correctly.  With WIN32 undefined, almost
> all packages build OOTB.  Simple as that.  Shouldn't we -- the users and
> distributors of Cygwin -- have the final say as to how Cygwin should be
> handled?
>

The problem is Cygwin has changed over time (IMO).  In the beginning 
when CMake started to support Cygwin we used as a way to build win32 
applications with a free compiler.  The charter for cygwin seems to have 
changed over the years to be a linux/unix system on top of windows where 
the default is to build X11 API applications.  For the past decade, the 
majority of VTK users of cygwin used the win32 api when using cygwin and 
not X11.

However, I can see the point of wanting unix/linux applications to build 
out of the box on cygwin with no changes.  Not sure I agree with using 
X11 on windows, but hey it is the quickest way to get an application 
running.

At the end of the day, CMake should be following the majority of the 
cygwin applications.  However, I really don't want to break the code of 
the people that have spent the time to get applications working on 
cygwin with CMake over the past 10 years.   That is what would happen if 
we suddenly changed CMake without warning.  The only applications that 
would break are the ones where people actually made an effort to port to 
cygwin using CMake.

So, I think the only way to fix this is to create a new policy.   The 
policy will warn all cygwin builds that CMake is no longer defining 
WIN32 on cygwin.  As with all policies, if a project has the minimum 
required CMake set to the version of CMake that implements the policy 
the new behavior will happen.  I know this will not make Yaakov totally 
happy as CMake based applications will not work unless they bump up the 
version of CMake they require, but over time all will be well.  The only 
other option that might make the cygwin X11 crowd happier would be to 
add a --cygwin-mode that forced the policy to be NEW, and would provide 
an easy way for Yaakov to port applications without having to change the 
source of the application being ported.

Some info on policies:
http://www.cmake.org/cmake/help/cmake-2-8-docs.html#section_Policies

-Bill