[CMake] Force out of source build

[hsanson]

Andreas Schneider wrote:
> hsanson wrote:
>> So how can I cause CMake to fail if I run it on the source directory??
>>
>
> Hi,
>
> try
>
> https://dev.csync.org/browser/cmake/Modules/MacroEnsureOutOfSourceBuild.cmake
Thanks,

The url above did not work for me but a Google search for 
MacroEnsureOutOfSourceBuild gave me what I needed.

>>
>> thanks
>> Horacio
>
>
>     -- andreas
>